More unconstify use

Replace casts whose only purpose is to cast away const with the
unconstify() macro.

Discussion: https://www.postgresql.org/message-id/flat/53a28052-f9f3-1808-fed9-460fd43035ab%402ndquadrant.com

src/backend/tcop/postgres.c

@@ -1767,7 +1767,7 @@ exec_bind_message(StringInfo input_message)
 				 * trailing null.  This is grotty but is a big win when
 				 * dealing with very large parameter strings.
 				 */
-				pbuf.data = (char *) pvalue;
+				pbuf.data = unconstify(char *, pvalue);
 				pbuf.maxlen = plength + 1;
 				pbuf.len = plength;
 				pbuf.cursor = 0;