Remove RPC TorchScript private API (#33978)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33978
We can directly pass user_callbale to rpc_async API in TorchScript. There is no need to have private API for taking qualified name.
ghstack-source-id: 99600360
Test Plan:
```
buck test mode/dev-nosan //caffe2/test/distributed/rpc/jit:rpc_fork
buck build mode/dev-nosan //caffe2/test/distributed/rpc/jit:rpc_fork \
&& buck-out/gen/caffe2/test/distributed/rpc/jit/rpc_fork\#binary.par -r test_torchscript_functions_not_supported
```
Differential Revision: D7420993
fbshipit-source-id: 228c15b21848e67418fab780e3fd6a1c6da5142d
