Remove gcc, gcc-c++ from private Koji build group

[Anthony Joseph Messina]

In preparation for F29, I went to define new build and srpm-build groups to
remove gcc an gcc-c++ and remain aligned with Fedora.

However, `koji add-group f29 build` does nothing--it doesn't add the group to
the database (associated with the f29 tag). I can also find no errors in the
Apache or PostgreSQL logs.

It's been a while since I've needed to modify groups which are typically
inherited from the previous version tag. Is there something I'm missing?