s390/purgatory: Remove unused '-MD' and unnecessary '-c' flags

When clang's -Qunused-arguments is dropped from KBUILD_CPPFLAGS, it
warns while building objects in the purgatory folder:

clang-16: error: argument unused during compilation: '-MD' [-Werror,-Wunused-command-line-argument]

'-MMD' is always passed to the preprocessor via c_flags, even when
KBUILD_CFLAGS is overridden in a folder, so clang complains the addition
of '-MD' will be unused. Remove '-MD' to clear up this warning, as it is
unnecessary with '-MMD'.

Additionally, '-c' is also unnecessary, remove it while in the area.

Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
Tested-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>