getdeps: remove the fb-mercurial-rust target
getdeps: remove the fb-mercurial-rust target

Summary:
Remove the standalone fb-mercurial-rust target that was an internal-only
dependency for the Eden build.  This build step is now done entirely in the
Eden build.

Reviewed By: fanzeyi

Differential Revision: D18623943

fbshipit-source-id: c62a1155ddd1c0a6b2270c472176ba25194c6145

facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst