lib/crc32: Remove unused combination support

Remove crc32_le_combine() and crc32_le_shift(), since they are no longer
used.

Although combination is an interesting thing that can be done with CRCs,
it turned out that none of the users of it in the kernel were even close
to being worthwhile.  All were much better off simply chaining the CRCs
or processing zeroes.

Let's remove the CRC32 combination code for now.  It can come back
(potentially optimized with carryless multiplication instructions) if
there is ever a case where it would actually be worthwhile.

Link: https://lore.kernel.org/r/20250607032228.27868-1-ebiggers@kernel.org


Signed-off-by: Eric Biggers <ebiggers@kernel.org>