Linear filtering depth is an odd operation that doesn't yield
particularly useful information.
In the primary instance where it'd be used, the GLES3.1 spec
(8.17) specifically denies its use, and enforcing the capability
can lead to issues with hardware designed without support in
mind.
We think the check isn't necessary as a result.

Bug: angleproject:5964
Change-Id: I69f243f7cc078cd5383de2cc4bf1cfc3b6f2bc3b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2894424Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>