Remove PG_SHA*_DIGEST_STRING_LENGTH from sha2.h

The last reference to those variables has been removed in aef8948f, so
this cleans up a bit the code.

Discussion: https://postgr.es/m/X//ggAqmTtt+3t7X@paquier.xyz

src/include/common/sha2.h

@@ -18,15 +18,11 @@
 /*** SHA224/256/384/512 Various Length Definitions ***********************/
 #define PG_SHA224_BLOCK_LENGTH			64
 #define PG_SHA224_DIGEST_LENGTH			28
-#define PG_SHA224_DIGEST_STRING_LENGTH	(PG_SHA224_DIGEST_LENGTH * 2 + 1)
 #define PG_SHA256_BLOCK_LENGTH			64
 #define PG_SHA256_DIGEST_LENGTH			32
-#define PG_SHA256_DIGEST_STRING_LENGTH	(PG_SHA256_DIGEST_LENGTH * 2 + 1)
 #define PG_SHA384_BLOCK_LENGTH			128
 #define PG_SHA384_DIGEST_LENGTH			48
-#define PG_SHA384_DIGEST_STRING_LENGTH	(PG_SHA384_DIGEST_LENGTH * 2 + 1)
 #define PG_SHA512_BLOCK_LENGTH			128
 #define PG_SHA512_DIGEST_LENGTH			64
-#define PG_SHA512_DIGEST_STRING_LENGTH	(PG_SHA512_DIGEST_LENGTH * 2 + 1)
 
 #endif							/* _PG_SHA2_H_ */