delete RCTHostCreationHelpers (#37703)

Summary:
Pull Request resolved: #37703

Changelog: [Internal]

i did too good of a job refactoring that these functions don't abstract any arguments away anymore, so let's just delete it

Reviewed By: javache

Differential Revision: D46303713

fbshipit-source-id: 7e0508a7296ebc3d5c62cf0824071bd2ef041193

Author: philIip