Revert "-D__ANDROID_NDK__ for SDK builds."

This reverts commit c324729692.

Reason for revert: defined in bionic instead (https://android-review.googlesource.com/c/platform/bionic/+/1135146).

Change-Id: Ia5d4d1b087cd43e14919a5791b44d4816ecbb434

cc/compiler.go

@@ -309,7 +309,6 @@ func (compiler *baseCompiler) compilerFlags(ctx ModuleContext, flags Flags, deps
 		flags.SystemIncludeFlags = append(flags.SystemIncludeFlags,
 			"-isystem "+getCurrentIncludePath(ctx).String(),
 			"-isystem "+getCurrentIncludePath(ctx).Join(ctx, config.NDKTriple(tc)).String())
-		flags.GlobalFlags = append(flags.GlobalFlags, "-D__ANDROID_NDK__")
 	}
 
 	if ctx.useVndk() {