-msse2 is passed by default in //build's gn setup (even -mssse3 now:
https://chromium-review.googlesource.com/c/chromium/src/+/2311044),
so there's no need to pass this explicitly.

Also remove some other things that are already set elsewhere.

No intended behavior change.

Bug: chromium:1220175
Change-Id: Id49540d17e21362e5f57156b47cf49a5534d89af
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/55610Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>