[iOS GPU] [easy] Rename APIs in MPSImageWrapper
Summary:
1. Clean up unused APIs on MPSImageWrapper
2. Rename textures to images to avoid confusions.
Test Plan: CI
Reviewed By: husthyc
Differential Revision: D28176917
fbshipit-source-id: 3afb261d9e5a9a6145ca3067cf0d245f1bf04683
