kbuild, x86: drop unnecessary prefix map configuration

The toplevel Makefile already provides -ffile-prefix-map as part of
KBUILD_CPPFLAGS. In contrast to the KBUILD_CFLAGS and KBUILD_AFLAGS
variables, KBUILD_CPPFLAGS is not redefined in the architecture specific
Makefiles. Therefore the toplevel KBUILD_CPPFLAGS do apply just fine, to
both C and ASM sources.

Drop the unnecessary custom prefix map configuration.

Link: https://lore.kernel.org/lkml/d250e864d6d81cc02e2599f710872f72d58a3c29.camel@decadent.org.uk/
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
2 files changed