GN: Don't explicitly define __ANDROID__ now that clang defines it automatically

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index d4a10c8899988225d8854c0c0d38affd6e74157c..80b13e6701dea860bb18cb0ee9a3dd1a907b7692 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -559,10 +559,6 @@ config("compiler") {
     }
 
     defines += [ "ANDROID" ]
-    if (is_clang) {
-      # GCC provides this as a built-in macro, but clang does not.
-      defines += [ "__ANDROID__" ]
-    }
 
     # The NDK has these things, but doesn't define the constants
     # to say that it does. Define them here instead.