kbuild: remove redundant extra warning flags

There is no point in turning individual options off and then on again, or
vice versa, as the last one always wins.  Now that -Wextra always gets
passed first, remove all the redundant lines about warnings that are
implied by either -Wall or -Wextra, and keep only the last one that
disables it in some configurations.

This should not have any effect but keep the Makefile more readable and
the command line shorter.

Link: https://lkml.kernel.org/r/20240415122037.1983124-3-arnd@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Kees Cook <keescook@chromium.org>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nicolas Schier <nicolas@fjasle.eu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>