remove use of folly::init (facebook#48573)

Summary:
Pull Request resolved: facebook#48573

changelog: [internal]

remove main.cpp for cxxreact/tests. It is not needed for tests to work and it breaks build for C++ only tests.

Reviewed By: javache

Differential Revision: D67975182

fbshipit-source-id: b9cbc5b5b6a87aafc69448e99877e664ed2d5af2

Author: sammy-SC