Remove function pointers in CopyTexture3DTest.
Remove function pointers in CopyTexture3DTest.

No longer needed with the ANGLE loader.

Bug: None
Change-Id: If206c529efaee5b81b68059341dcc6e841126842
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1556698
Commit-Queue: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>

ppy/angle,ppy/angle,ppy/angle,ppy/angle