The spec for OES_shader_multisample_interpolation requires
support for OES_sample_variables. Disable support until
the changes related to OES_sample_variables land.

Bug: angleproject:3589
Bug: angleproject:3588
Change-Id: I94408a4f4fd74c19922c011242a515cc457910cf
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2535075Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>