Makefile: drop arch/*/include/asm/proc from make mrproper pattern

Commit 7d89982b stopped creating symbolic link
arch/${arch}/include/asm/proc.

We do not need to delete it by "make mrproper" any more.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Vasili Galka <vvv444@gmail.com>

Makefile

@@ -1274,7 +1274,7 @@ $(mrproper-dirs):
 mrproper: clobber $(mrproper-dirs)
 	$(call cmd,rmdirs)
 	$(call cmd,rmfiles)
-	@rm -f arch/*/include/asm/arch arch/*/include/asm/proc
+	@rm -f arch/*/include/asm/arch
 
 # distclean
 #