Remove unused forward declaration (#30154)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30154
This doesn't seem to be used in thread_pool.cpp.
ghstack-source-id: 94264158
Test Plan: Let's see if this compiles.
Differential Revision: D18614141
fbshipit-source-id: c6ff3db56b55fcee7d8123d909ee275690163ece
