Angle: Remove obsolete requires_sdk_api_level_23 from BUILD.gn.

Will be removed upstream.

BUG=chromium:688465
Change-Id: If50ac3624a5f692755797eaabcc0c7d6e7408655
Reviewed-on: https://chromium-review.googlesource.com/1244079
Reviewed-by: Yuly Novikov <ynovikov@google.com>
Commit-Queue: Yuly Novikov <ynovikov@google.com>

BUILD.gn

@@ -968,10 +968,6 @@ if (is_android) {
 
     android_manifest = "AndroidManifest.xml"
 
-    # requires_sdk_api_level_23 tells the apk tool that this apk is only
-    # supported on M or later releases (which is true for Vulkan).
-    # Allows tool to optimize for that.
-    requires_sdk_api_level_23 = true
     apk_name = "ANGLEPrebuilt"
     uncompress_shared_libraries = true
     shared_libraries = [