kbuild: remove cc-option test of -Werror=date-time

The minimal compiler versions, GCC 4.9 and Clang 10 support this flag.

Here is the godbolt:
https://godbolt.org/z/xvjcMa



Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Acked-by: Will Deacon <will@kernel.org>