Switch cargo build back to default system ld

Summary: If what this comment says is true, `lld` would no longer be required. The zstd-rs fork is gone since {D50419456} + {D50394320}.

Reviewed By: zertosh

Differential Revision: D50480876

fbshipit-source-id: 95c8bca9bd23c158cc352edfd3776ac5b8d3d41c

build/fbcode_builder/manifests/mononoke

@@ -17,7 +17,6 @@ builder = nop
 [cargo]
 build_doc = true
 workspace_dir = eden/mononoke
-cargo_config_file = source/eden/mononoke/.cargo/config
 
 [shipit.pathmap]
 fbcode/configerator/structs/scm/hg = configerator/structs/scm/hg
@@ -48,8 +47,5 @@ fb303
 fbthrift
 rust-shed
 
-[dependencies.os=linux]
-lld
-
 [dependencies.fb=on]
 rust