Fix incorrect comments in hmac.c and hmac_openssl.c

Both files referred to pg_hmac_ctx->data, which, I guess, comes from the
early versions of the patch that has resulted in commit e6bdfd9.

Author: Sergey Shinderuk
Discussion: https://postgr.es/m/8cbb56dd-63d6-a581-7a65-25a97ac4be03@postgrespro.ru
Backpatch-through: 14

Author: michaelpq

src/common/hmac.c

@@ -38,9 +38,7 @@
 #define FREE(ptr) free(ptr)
 #endif
 
-/*
- * Internal structure for pg_hmac_ctx->data with this implementation.
- */
+/* Internal pg_hmac_ctx structure */
 struct pg_hmac_ctx
 {
 	pg_cryptohash_ctx *hash;

src/common/hmac_openssl.c

@@ -50,9 +50,7 @@
 #define FREE(ptr) free(ptr)
 #endif							/* FRONTEND */
 
-/*
- * Internal structure for pg_hmac_ctx->data with this implementation.
- */
+/* Internal pg_hmac_ctx structure */
 struct pg_hmac_ctx
 {
 	HMAC_CTX   *hmacctx;