[easy][antlir] more dead code in build_defs

Summary: You gotta be kidding me

Test Plan:
```
❯ buck targets fbcode//antlir/...
```

Differential Revision: D53968022

antlir/bzl/build_defs.bzl

@@ -265,7 +265,6 @@ is_buck2 = shim.is_buck2
 is_facebook = shim.is_facebook
 get_cxx_platform_for_current_buildfile = shim.get_cxx_platform_for_current_buildfile
 do_not_use_repo_cfg = shim.do_not_use_repo_cfg
-rpm_vset = shim.rpm_vset
 repository_name = shim.repository_name
 target_utils = shim.target_utils
 alias = shim.alias
@@ -275,4 +274,3 @@ third_party = struct(
     source = shim.third_party.source,
     libraries = _third_party_libraries,
 )
-thrift_library = shim.thrift_library