fix(coreos-base/gmerge): Remove reference to old package.provided file

A case of binary packages masking breakage, didn't notice this broke
because I didn't happen to trigger a build of gmerge during my testing.
This package.provided file contained the hackily installed toolchain
which is now handled via a normal emerge instead.
Merge pull request #273 from marineam/gmerge

fix(coreos-base/gmerge): Remove reference to old package.provided file