A few built-in variables are checking against overly complicated
extension conditions, deleting these will make the array lookup
easier to implement.

Also changed functionality to always enable GL_OVR_multiview
when GL_OVR_multiview2 is enabled

Bug: angleproject:3805
Change-Id: I10fc7db0fa8b496fbba8b2c27a7311ac3c24dbe4
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1752082
Commit-Queue: Clemen Deng <clemendeng@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>