Remove the UNIMPLEMENTED() macro. Doing nothing is an acceptable
implementation at this time.

Test: Angry Birds 2
Bug: b/154930384
Change-Id: Ie46c084feb2015aa5fecdbbf2295d46e5087e3dc
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2172711
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>