Remove hostcaps fb feature

Summary:
`hostcaps`'s `fb` feature enables getting the real `fbwhoami` values
during cargo builds.

The only cargo-based projects using `hostcaps` are
`eden/scm/lib/commitcloudsubscriber` and `eden/scm/lib/config/loader`.
Neither is making internal builds with `cargo`, so neither the `fb`
feature nor `fbwhoami` are actually used.

The `fbwhoami` refs in `eden/mononoke/rate_limiting`
`eden/mononoke/server/qps` are from internal code. So that's not used
either.

The only thing using `fbwhoami` from a cargo build that needs it is
`common/rust/scuba` and `scm/telemetry/scm-telem-log` - so both for
`scm_telemetry`.

Reviewed By: dtolnay

Differential Revision: D52216174

fbshipit-source-id: 8c97e20d2eb0dd3152503d40e4796656b932d2a0

build/fbcode_builder/manifests/eden

@@ -58,7 +58,6 @@ lmdb
 [shipit.pathmap.fb=on]
 # for internal builds that use getdeps
 fbcode/fb303 = fb303
-fbcode/common/rust/fbwhoami = common/rust/fbwhoami
 fbcode/common/rust/shed = common/rust/shed
 fbcode/thrift/lib/rust = thrift/lib/rust