Remove unnecessary flags from BUILD.gn
Remove unnecessary flags from BUILD.gn

The GN build gets many default flags from //build/config. Remove
flags that are redundant with the default flags.

No intended behavior change (except for the hash style).

Bug: chromium:1220175
Change-Id: Idcebc8f55165ae300632005f5cf527ec689a365f
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/55608
Reviewed-by: Alexis Hétu <1c7141772178a7e80858c3db5052b328076f08f4@google.com>
Kokoro-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Tested-by: Nico Weber <c0f02d98506190746d1cdcb01be0f37b51e27077@chromium.org>
Commit-Queue: Nico Weber <c0f02d98506190746d1cdcb01be0f37b51e27077@chromium.org>

google/swiftshader,google/swiftshader,google/swiftshader