kernel.bbclass: remove unnecessary dead code
The grep pattern seems to have been wrong ever since we stopped adding
the -ffile-prefix-map via a patch in commit
20aea61385e, because the
actual upstream gcc produces
-ffile-prefix-map=<old>=<new>
and not
-ffile-prefix-map=<old=new>
Besides, these *-prefix-map options are already used when building the
kernel due to the setting of KERNEL_CC, and the kernel build system
itself also probes for and uses at least -fmacro-prefix-map (hence
taking care of __FILE__ etc., but not necessarily things that go in
.debug_info sections).
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
