Remove kOverrideUseSoftwareGLForTests.

Instead, we should use --use-gl=angle --use-angle=swiftshader for
tests that want to use SwiftShader instead of real GPU, for example, tests run on bots without real GPUs, but still want to exercise gpu accelerated paths instead of software fallback paths.

Bug:1426236
TEST=bots

Change-Id: If6d7e88fb3bf626f56f8f8d75e9d5ad6b13f1768
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4355711


Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/main@{#1120020}