chromium: Build with GN unconditionally

Previously I've added the extra file common-gn.nix in addition to
common.nix, so we can possibly have a smooth transition from current
stable to the new version 54.

Unfortunately, version 53 is already EOL and we have to move to version
54 as soon as possible so we can only use GN and thus it doesn't make
sense to provide expressions for GYP anymore.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>