kbuild: remove KBUILD_ENABLE_EXTRA_GCC_CHECKS support

Commit e27128db ("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>