Remove PG_SHA*_DIGEST_STRING_LENGTH from sha2.h

On Thu, Jan 14, 2021 at 03:11:12PM +0900, Michael Paquier wrote:

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.

Cleanup done as of ccf4e27.
--
Michael