boards/native: remove toolchain variables duplication
boards/native: remove toolchain variables duplication

The toolchain variables were overwritten anyway in
`makefiles/toolchain/gnu|llvm.inc.mk` as they use `=` affectation,
except for `OBJCOPY`.

On my Linux ubuntu bionic machine, the only difference for `info-build`
with both gnu/llvm and also with LTO=1
is that it now uses an absolute path for `objcopy`

    -OBJCOPY: objcopy
    +OBJCOPY: /usr/bin/objcopy

On the OSX 17.7.0 I tested, there was no difference.

yogo1212/RIOT,x3ro/RIOT,aeneby/RIOT,basilfx/RIOT,rfuentess/RIOT,OlegHahm/RIOT,cladmi/RIOT,rfuentess/RIOT,cladmi/RIOT,jasonatran/RIOT,aeneby/RIOT,rfuentess/RIOT,yogo1212/RIOT,x3ro/RIOT,mtausig/RIOT,mtausig/RIOT,ant9000/RIOT,basilfx/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,smlng/RIOT,OTAkeys/RIOT,authmillenon/RIOT,mtausig/RIOT,basilfx/RIOT,OTAkeys/RIOT,rfuentess/RIOT,kYc0o/RIOT,miri64/RIOT,mfrey/RIOT,jasonatran/RIOT,josephnoir/RIOT,RIOT-OS/RIOT,cladmi/RIOT,authmillenon/RIOT,toonst/RIOT,toonst/RIOT,rfuentess/RIOT,x3ro/RIOT,yogo1212/RIOT,authmillenon/RIOT,yogo1212/RIOT,yogo1212/RIOT,aeneby/RIOT,mfrey/RIOT,mfrey/RIOT,cladmi/RIOT,x3ro/RIOT,smlng/RIOT,toonst/RIOT,miri64/RIOT,OlegHahm/RIOT,aeneby/RIOT,smlng/RIOT,mfrey/RIOT,mfrey/RIOT,yogo1212/RIOT,kYc0o/RIOT,jasonatran/RIOT,miri64/RIOT,josephnoir/RIOT,toonst/RIOT,cladmi/RIOT,OTAkeys/RIOT,mtausig/RIOT,josephnoir/RIOT,kYc0o/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,mtausig/RIOT,jasonatran/RIOT,smlng/RIOT,ant9000/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,josephnoir/RIOT,basilfx/RIOT,OTAkeys/RIOT,miri64/RIOT,ant9000/RIOT,kaspar030/RIOT,x3ro/RIOT,ant9000/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,ant9000/RIOT,OlegHahm/RIOT,kaspar030/RIOT,kYc0o/RIOT,kaspar030/RIOT,toonst/RIOT,jasonatran/RIOT,miri64/RIOT,kYc0o/RIOT,josephnoir/RIOT,OlegHahm/RIOT,authmillenon/RIOT,smlng/RIOT,basilfx/RIOT,aeneby/RIOT,RIOT-OS/RIOT,authmillenon/RIOT