kbuild: drop clang-specific extra warnings

It is true clang has more warnings enabled by default but
Jeroen Hofstee are making lots of efforts to eliminate warnings.
Let's drop clang-related checks and continue to fix C code.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Jeroen Hofstee <jeroen@myspectrum.nl>
kbuild: remove "clobber" target

Now we stick to Linux Kernel's build system, so squash
"clobber" to "clean" target.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>