Remove PG_SHA*_DIGEST_STRING_LENGTH from sha2.h

Hi all,

I have just noticed that aef8948 has removed the last reference to
PG_SHA256_DIGEST_STRING_LENGTH in the code (this was getting used to
know the length of a SHA256 digest encoded to hex for checksum
manifests, but pg_hex_enc_len() calculates the same in a more
consistent way).  Attached is a patch to remove all those now-useless
declarations, for all four SHA2 options.

Thoughts?
--
Michael