Revert commit cebc04ba ("add CONFIG_ENABLE_MUST_CHECK").

A lot of warn_unused_result warnings existed in 2006, but until now
they have been fixed thanks to people doing allmodconfig tests.

Our goal is to always enable __must_check where appropriate, so this
CONFIG option is no longer needed.

I see a lot of defconfig (arch/*/configs/*_defconfig) files having:

    # CONFIG_ENABLE_MUST_CHECK is not set

I did not touch them for now since it would be a big churn. If arch
maintainers want to clean them up, please go ahead.

While I was here, I also moved __must_check to compiler_attributes.h
from compiler_types.h
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Jason A. Donenfeld <Jason@zx2c4.com>
Acked-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
[Moved addition in compiler_attributes.h to keep it sorted]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>