kbuild: remove KBUILD_ENABLE_EXTRA_GCC_CHECKS support

Commit e27128d ("kbuild: rename KBUILD_ENABLE_EXTRA_GCC_CHECKS to
KBUILD_EXTRA_WARN") renamed KBUILD_ENABLE_EXTRA_GCC_CHECKS in 2019.
The migration in downstream code should be complete.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>

Author: masahir0y

Makefile

@@ -151,9 +151,6 @@ endif
 
 export KBUILD_EXTMOD
 
-# backward compatibility
-KBUILD_EXTRA_WARN ?= $(KBUILD_ENABLE_EXTRA_GCC_CHECKS)
-
 ifeq ("$(origin W)", "command line")
   KBUILD_EXTRA_WARN := $(W)
 endif