imagebuildah.Executor: remove unused onbuild field
imagebuildah.Executor: remove unused onbuild field

Reported by golangci-lint.

Signed-off-by: Valentin Rothberg <rothberg@redhat.com>

Closes: #1624
Approved by: TomSweeneyRedHat

rhatdan/buildah,nalind/buildah-1,projectatomic/buildah,projectatomic/buildah,rhatdan/buildah,TomSweeneyRedHat/buildah,projectatomic/buildah,nalind/buildah,TomSweeneyRedHat/buildah,nalind/buildah-1,nalind/buildah-1,TomSweeneyRedHat/buildah,rhatdan/buildah,nalind/buildah