boards/nucleo-f103: remove STDIO_UART definition
boards/nucleo-f103: remove STDIO_UART definition

The board is using the default value, so no need to
redefine it...

BytesGalore/RIOT,kaleb-himes/RIOT,toonst/RIOT,kYc0o/RIOT,hamilton-mote/RIOT-OS,adjih/RIOT,LudwigOrtmann/RIOT,smlng/RIOT,authmillenon/RIOT,cladmi/RIOT,x3ro/RIOT,neiljay/RIOT,kaspar030/RIOT,miri64/RIOT,aeneby/RIOT,Josar/RIOT,mtausig/RIOT,ks156/RIOT,neiljay/RIOT,ks156/RIOT,miri64/RIOT,jfischer-phytec-iot/RIOT,kaspar030/RIOT,miri64/RIOT,yogo1212/RIOT,mtausig/RIOT,OTAkeys/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,ant9000/RIOT,ks156/RIOT,gebart/RIOT,josephnoir/RIOT,beurdouche/RIOT,aeneby/RIOT,dailab/RIOT,beurdouche/RIOT,kbumsik/RIOT,immesys/RiSyn,Hyungsin/RIOT-OS,katezilla/RIOT,OlegHahm/RIOT,ks156/RIOT,kaleb-himes/RIOT,adrianghc/RIOT,avmelnikoff/RIOT,tfar/RIOT,neumodisch/RIOT,adrianghc/RIOT,plushvoxel/RIOT,biboc/RIOT,lazytech-org/RIOT,immesys/RiSyn,tfar/RIOT,jasonatran/RIOT,cladmi/RIOT,josephnoir/RIOT,toonst/RIOT,dailab/RIOT,kb2ma/RIOT,gebart/RIOT,hamilton-mote/RIOT-OS,gautric/RIOT,gautric/RIOT,rfuentess/RIOT,plushvoxel/RIOT,kbumsik/RIOT,dkm/RIOT,alignan/RIOT,gebart/RIOT,backenklee/RIOT,Hyungsin/RIOT-OS,dkm/RIOT,LudwigKnuepfer/RIOT,toonst/RIOT,roberthartung/RIOT,toonst/RIOT,kaspar030/RIOT,basilfx/RIOT,mfrey/RIOT,neiljay/RIOT,A-Paul/RIOT,backenklee/RIOT,kerneltask/RIOT,kerneltask/RIOT,OTAkeys/RIOT,jbeyerstedt/RIOT-OTA-update,gautric/RIOT,kerneltask/RIOT,dkm/RIOT,avmelnikoff/RIOT,TobiasFredersdorf/RIOT,roberthartung/RIOT,plushvoxel/RIOT,kerneltask/RIOT,OlegHahm/RIOT,cladmi/RIOT,alignan/RIOT,mtausig/RIOT,thomaseichinger/RIOT,josephnoir/RIOT,avmelnikoff/RIOT,ant9000/RIOT,Hyungsin/RIOT-OS,syin2/RIOT,lazytech-org/RIOT,jasonatran/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,RIOT-OS/RIOT,avmelnikoff/RIOT,Josar/RIOT,aeneby/RIOT,katezilla/RIOT,tfar/RIOT,plushvoxel/RIOT,aeneby/RIOT,smlng/RIOT,backenklee/RIOT,biboc/RIOT,jbeyerstedt/RIOT-OTA-update,tfar/RIOT,immesys/RiSyn,rfuentess/RIOT,aeneby/RIOT,smlng/RIOT,smlng/RIOT,ant9000/RIOT,yogo1212/RIOT,basilfx/RIOT,rfuentess/RIOT,LudwigKnuepfer/RIOT,Hyungsin/RIOT-OS,rfuentess/RIOT,A-Paul/RIOT,yogo1212/RIOT,Josar/RIOT,katezilla/RIOT,RIOT-OS/RIOT,roberthartung/RIOT,biboc/RIOT,neumodisch/RIOT,TobiasFredersdorf/RIOT,biboc/RIOT,miri64/RIOT,syin2/RIOT,lazytech-org/RIOT,alignan/RIOT,syin2/RIOT,thomaseichinger/RIOT,LudwigOrtmann/RIOT,Ell-i/RIOT,adrianghc/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,A-Paul/RIOT,BytesGalore/RIOT,x3ro/RIOT,dkm/RIOT,Josar/RIOT,lazytech-org/RIOT,x3ro/RIOT,yogo1212/RIOT,beurdouche/RIOT,neumodisch/RIOT,kbumsik/RIOT,adrianghc/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,jasonatran/RIOT,jbeyerstedt/RIOT-OTA-update,jfischer-phytec-iot/RIOT,jfischer-phytec-iot/RIOT,jasonatran/RIOT,toonst/RIOT,mfrey/RIOT,BytesGalore/RIOT,jbeyerstedt/RIOT-OTA-update,kbumsik/RIOT,mfrey/RIOT,immesys/RiSyn,syin2/RIOT,thomaseichinger/RIOT,A-Paul/RIOT,OTAkeys/RIOT,gebart/RIOT,adjih/RIOT,authmillenon/RIOT,lazytech-org/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,kb2ma/RIOT,avmelnikoff/RIOT,gautric/RIOT,jasonatran/RIOT,jfischer-phytec-iot/RIOT,A-Paul/RIOT,Hyungsin/RIOT-OS,kaleb-himes/RIOT,tfar/RIOT,josephnoir/RIOT,kb2ma/RIOT,kYc0o/RIOT,basilfx/RIOT,jbeyerstedt/RIOT-OTA-update,mtausig/RIOT,ant9000/RIOT,OTAkeys/RIOT,josephnoir/RIOT,ant9000/RIOT,dailab/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,mfrey/RIOT,BytesGalore/RIOT,OlegHahm/RIOT,kaleb-himes/RIOT,kbumsik/RIOT,Josar/RIOT,rfuentess/RIOT,LudwigKnuepfer/RIOT,authmillenon/RIOT,kb2ma/RIOT,yogo1212/RIOT,yogo1212/RIOT,LudwigOrtmann/RIOT,mfrey/RIOT,mtausig/RIOT,RIOT-OS/RIOT,Ell-i/RIOT,x3ro/RIOT,plushvoxel/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,hamilton-mote/RIOT-OS,Ell-i/RIOT,jfischer-phytec-iot/RIOT,beurdouche/RIOT,kerneltask/RIOT,hamilton-mote/RIOT-OS,gautric/RIOT,x3ro/RIOT,kb2ma/RIOT,miri64/RIOT,backenklee/RIOT,authmillenon/RIOT,adjih/RIOT,alignan/RIOT,OlegHahm/RIOT,neiljay/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,ks156/RIOT,immesys/RiSyn,neumodisch/RIOT,dkm/RIOT,TobiasFredersdorf/RIOT,LudwigOrtmann/RIOT,roberthartung/RIOT,Ell-i/RIOT,immesys/RiSyn,LudwigKnuepfer/RIOT,authmillenon/RIOT,kaspar030/RIOT,neumodisch/RIOT,backenklee/RIOT,gebart/RIOT,kYc0o/RIOT,thomaseichinger/RIOT,dailab/RIOT,katezilla/RIOT,adjih/RIOT,TobiasFredersdorf/RIOT,LudwigKnuepfer/RIOT,BytesGalore/RIOT,adjih/RIOT,cladmi/RIOT,neumodisch/RIOT,TobiasFredersdorf/RIOT,thomaseichinger/RIOT,basilfx/RIOT,hamilton-mote/RIOT-OS,dailab/RIOT,authmillenon/RIOT,roberthartung/RIOT,adrianghc/RIOT,smlng/RIOT,kaleb-himes/RIOT,basilfx/RIOT,LudwigOrtmann/RIOT,katezilla/RIOT,Ell-i/RIOT,syin2/RIOT,biboc/RIOT,neiljay/RIOT,beurdouche/RIOT,LudwigOrtmann/RIOT,kYc0o/RIOT,alignan/RIOT,cladmi/RIOT