ftrace: use the new kbuild CFLAGS_REMOVE for kernel directory

This patch removes the Makefile turd and uses the nice CFLAGS_REMOVE macro
in the kernel directory.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>