Clean up the AVX2 activation in the gni file

Bug: webrtc:11663
Change-Id: Ib85ee67c72175270ddf212c7f1a09115906f0c4c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/185480
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Per Åhgren <peah@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32202}

webrtc.gni

@@ -243,11 +243,7 @@ declare_args() {
 
   # Set this to true to enable the avx2 support in webrtc.
   # TODO: Make sure that AVX2 works also for non-clang compilers.
-  if (is_clang == true) {
-    rtc_enable_avx2 = true
-  } else {
-    rtc_enable_avx2 = false
-  }
+  rtc_enable_avx2 = is_clang
 
   # Include tests in standalone checkout.
   rtc_include_tests = !build_with_chromium && !build_with_mozilla