lib/crc32: remove unused support for CRC32C combination

crc32c_combine() and crc32c_shift() are no longer used (except by the
KUnit test that tests them), and their current implementation is very
slow.  Remove them.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Link: https://patch.msgid.link/20250519175012.36581-8-ebiggers@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>