The DIGEST_FOR_NULL_VALUES feature was added in 21a77612b3 (2020; 4.4)
and can now be assumed to be always present. The hasher which it invoked
is removed.
The XXHASH feature was introduced in 0bab3e59c2 (2017; 2.2) and made
mandatory in defe6f49df (2020; 4.4), but some vestiges remain.
Remove them now. Note that md5_hasher itself is still in use by
other components, so it cannot be removed.