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

build/fbcode_builder/manifests/eden

@@ -38,7 +38,6 @@ libcurl
 
 [dependencies.fb=on]
 fb-mercurial
-fb-mercurial-rust
 
 [shipit.pathmap]
 fbcode/eden/oss = .