Remove SHA256_HMAC_B from scram-common.h

This referred to the size of the buffers for k_ipad and k_opad in HMAC
computations.  This is unused since e6bdfd9, where SCRAM has switched to
the cryptohash routines for its HMAC calculations rather than its own
maths.

Reviewed-by: Jacob Champion
Discussion: https://postgr.es/m/Y5gGMjXhyp0oK0mH@paquier.xyz

Author: michaelpq

src/include/common/scram-common.h

@@ -23,9 +23,6 @@
 /* Length of SCRAM keys (client and server) */
 #define SCRAM_KEY_LEN				PG_SHA256_DIGEST_LENGTH
 
-/* length of HMAC */
-#define SHA256_HMAC_B				PG_SHA256_BLOCK_LENGTH
-
 /*
  * Size of random nonce generated in the authentication exchange.  This
  * is in "raw" number of bytes, the actual nonces sent over the wire are