Removes the EGL 1.5 overrides now that they are upstreamed.

Bug: angleproject:3446
Change-Id: I4b17354518534ea821bba83a4b1e00f83f80ac37
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1605786Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>