crypto: Clean up help text for CRYPTO_CRC32C

crypto: Clean up help text for CRYPTO_CRC32C

Ext4, jbd2, iSCSI, NVMeoF/TCP, and Btrfs stopped using this CRC32c
implementation in commits f2b4fa19647e18a2 ("ext4: switch to using the
crc32c library"), dd348f054b24a3f5 ("jbd2: switch to using the crc32c
library"), 92186c1455a2d356 ("scsi: iscsi_tcp: Switch to using the
crc32c library"), 427fff9aff295e2c ("nvme-tcp: use crc32c() and
skb_copy_and_crc32c_datagram_iter()"), and fe11ac191ce0ad91 ("btrfs:
switch to library APIs for checksums").

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/f567add7840bc612382237b3e76f3a8bdbd671e6.1772116160.git.geert+renesas@glider.be
Signed-off-by: Eric Biggers <ebiggers@kernel.org>