Remove unnecessary USE_CLANG_PLATFORM_BUILD.

This build variable is unnecessary now that GCC is completely
unsupported for Android platform builds.

Bug: http://b/64032869
Test: Builds
Change-Id: I9d44ebd7129cb2bdfbb26d37922db19c6fb9efc9
diff --git a/Changes.md b/Changes.md
index 3e48bad..37bbad0 100644
--- a/Changes.md
+++ b/Changes.md
@@ -92,6 +92,11 @@
 attribute to the root element `<manifest>`. If `PRODUCT_COMPATIBILITY_MATRIX_LEVEL_OVERRIDE`
 is 26 or 27, you can add `"target-level"="1"` to your device manifest instead.
 
+### Stop using USE_CLANG_PLATFORM_BUILD {#USE_CLANG_PLATFORM_BUILD}
+
+Clang is the default and only supported Android compiler, so there is no reason
+for this option to exist.
+
 ### Other envsetup.sh variables  {#other_envsetup_variables}
 
 * ANDROID_TOOLCHAIN