Don't install hg-git anymore

Summary:
Not maintained on FreeBSD.

Still maintained at https://hg-git.github.io/ but as there is no current
objective to convert repositories, it doesn't need to be repackaged right now.

Reviewers: DorianWinty

Reviewed By: DorianWinty

Differential Revision: https://devcentral.nasqueron.org/D3449