lib: Removed MPILIB, MPILIB_EXTRA, and SIGNATURE prompts

As modules are expected to select MPILIB, MPILIB_EXTRA, and SIGNATURE,
removed Kconfig prompts.

Requested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dmitry Kasatkin <dmitry.kasatkin@intel.com>
Signed-off-by: James Morris <jmorris@namei.org>
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