x86/build: Drop unused mflags-y

Subarchitecture support (mflags-y) was removed from x86 in this commit:

   6bda2c8b ("x86: remove subarchitecture support")

So drop the mflags-y usage from the Makefile.
Signed-off-by: Cao jin <caoj.fnst@cn.fujitsu.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1502105384-23214-1-git-send-email-caoj.fnst@cn.fujitsu.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/Makefile

@@ -232,9 +232,6 @@ KBUILD_CFLAGS += -Wno-sign-compare
 #
 KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
 
-KBUILD_CFLAGS += $(mflags-y)
-KBUILD_AFLAGS += $(mflags-y)
-
 archscripts: scripts_basic
 	$(Q)$(MAKE) $(build)=arch/x86/tools relocs