Fix old, misleading comment for PGRES_POLLING_ACTIVE.

The comment implies that we can eventually remove this, but per
discussion, we actually don't want to do that ever, in order to
maintain compatibility.

Jelte Fennema-Nio, reviewed by Tristan Partin

Discussion: http://postgr.es/m/CAGECzQTO72jKed5461W8cytV2Msh_e+WUZjOyX_RUQCbjk4LRA@mail.gmail.com

src/interfaces/libpq/libpq-fe.h

@@ -91,8 +91,7 @@ typedef enum
 	PGRES_POLLING_READING,		/* These two indicate that one may	  */
 	PGRES_POLLING_WRITING,		/* use select before polling again.   */
 	PGRES_POLLING_OK,
-	PGRES_POLLING_ACTIVE		/* unused; keep for awhile for backwards
-								 * compatibility */
+	PGRES_POLLING_ACTIVE		/* unused; keep for backwards compatibility */
 } PostgresPollingStatusType;
 
 typedef enum