RFR: JDK-8199125: --disable-warnings-as-errors does not work for native jtreg test code

[Erik Joelsson]

In TestFilesCompilation.gmk we still add $(CFLAGS_WARNINGS_ARE_ERRORS) 
explicitly to CFLAGS. This is both redundant and wrong since we are now 
handling these flags automatically in the macro SetupNativeCompilation. 
The consequence is that --disable-warnings-as-errors won't work for 
jtreg tests.

Bug: https://bugs.openjdk.java.net/browse/JDK-8196458
Webrev: http://cr.openjdk.java.net/~erikj/8199125/webrev.01/

/Erik