Remove useless pg_strdup() operations.

split_to_stringlist() doesn't modify its first argument nor expect it
to remain valid after exit, so there's no need to duplicate the optarg
string at the call sites.  Per Coverity.  (This has been wrong all along,
but commit 052cc223 changed the useless calls from "strdup" to
"pg_strdup", which apparently made Coverity think it's a new bug.
It's not, but it's also not worth back-patching.)

src/test/regress/pg_regress.c

@@ -2064,7 +2064,7 @@ regression_main(int argc, char *argv[], init_function ifunc, test_function tfunc
 				 * before we add the specified one.
 				 */
 				free_stringlist(&dblist);
-				split_to_stringlist(pg_strdup(optarg), ", ", &dblist);
+				split_to_stringlist(optarg, ", ", &dblist);
 				break;
 			case 2:
 				debug = true;
@@ -2114,7 +2114,7 @@ regression_main(int argc, char *argv[], init_function ifunc, test_function tfunc
 				dlpath = pg_strdup(optarg);
 				break;
 			case 18:
-				split_to_stringlist(pg_strdup(optarg), ", ", &extraroles);
+				split_to_stringlist(optarg, ", ", &extraroles);
 				break;
 			case 19:
 				add_stringlist_item(&temp_configs, optarg);