gnrc_sixlowpan: remove assertion on multiple_by_size function
gnrc_sixlowpan: remove assertion on multiple_by_size function

During the review of #9485 [we found out][1] that an assertion in this
function was invalid. However, the documentation on this assertion
wasn't removed on that. This fixes that.

[1] https://github.com/RIOT-OS/RIOT/pull/9485#issuecomment-407794280

BytesGalore/RIOT,kaspar030/RIOT,aeneby/RIOT,ant9000/RIOT,biboc/RIOT,toonst/RIOT,RIOT-OS/RIOT,avmelnikoff/RIOT,authmillenon/RIOT,miri64/RIOT,A-Paul/RIOT,yogo1212/RIOT,aeneby/RIOT,ant9000/RIOT,toonst/RIOT,basilfx/RIOT,basilfx/RIOT,mtausig/RIOT,gebart/RIOT,smlng/RIOT,toonst/RIOT,aeneby/RIOT,kYc0o/RIOT,biboc/RIOT,rfuentess/RIOT,josephnoir/RIOT,yogo1212/RIOT,jasonatran/RIOT,kYc0o/RIOT,ant9000/RIOT,ant9000/RIOT,kYc0o/RIOT,smlng/RIOT,yogo1212/RIOT,BytesGalore/RIOT,OlegHahm/RIOT,yogo1212/RIOT,mfrey/RIOT,x3ro/RIOT,A-Paul/RIOT,avmelnikoff/RIOT,lazytech-org/RIOT,biboc/RIOT,BytesGalore/RIOT,miri64/RIOT,OTAkeys/RIOT,mtausig/RIOT,cladmi/RIOT,BytesGalore/RIOT,basilfx/RIOT,mfrey/RIOT,aeneby/RIOT,BytesGalore/RIOT,cladmi/RIOT,jasonatran/RIOT,cladmi/RIOT,kbumsik/RIOT,mfrey/RIOT,biboc/RIOT,kbumsik/RIOT,jasonatran/RIOT,basilfx/RIOT,kaspar030/RIOT,kbumsik/RIOT,OlegHahm/RIOT,mtausig/RIOT,authmillenon/RIOT,authmillenon/RIOT,miri64/RIOT,rfuentess/RIOT,aeneby/RIOT,kaspar030/RIOT,x3ro/RIOT,miri64/RIOT,lazytech-org/RIOT,jasonatran/RIOT,cladmi/RIOT,josephnoir/RIOT,smlng/RIOT,mfrey/RIOT,OlegHahm/RIOT,rfuentess/RIOT,gebart/RIOT,yogo1212/RIOT,A-Paul/RIOT,authmillenon/RIOT,cladmi/RIOT,toonst/RIOT,josephnoir/RIOT,authmillenon/RIOT,OTAkeys/RIOT,avmelnikoff/RIOT,avmelnikoff/RIOT,gebart/RIOT,mtausig/RIOT,OlegHahm/RIOT,josephnoir/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,A-Paul/RIOT,gebart/RIOT,ant9000/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,josephnoir/RIOT,OTAkeys/RIOT,rfuentess/RIOT,kbumsik/RIOT,lazytech-org/RIOT,OTAkeys/RIOT,x3ro/RIOT,kaspar030/RIOT,miri64/RIOT,rfuentess/RIOT,authmillenon/RIOT,x3ro/RIOT,lazytech-org/RIOT,smlng/RIOT,kYc0o/RIOT,A-Paul/RIOT,jasonatran/RIOT,x3ro/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,basilfx/RIOT,OTAkeys/RIOT,smlng/RIOT,kbumsik/RIOT,gebart/RIOT,mfrey/RIOT,avmelnikoff/RIOT,RIOT-OS/RIOT,mtausig/RIOT,toonst/RIOT,kaspar030/RIOT,biboc/RIOT,lazytech-org/RIOT