bcachefs: Remove unnecessary softdep on xxhash

As with the other algorithms, bcachefs does not access xxhash through
the crypto API.  So there is no need to use a module softdep to ensure
that it is loaded.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>