Remove silly completion for "DELETE FROM tabname ...".

psql offered USING, WHERE, and SET in this context, but SET is not a valid
possibility here.  Seems to have been a thinko in commit f5ab0a14ea
which added DELETE's USING option.

src/bin/psql/tab-complete.c

@@ -2003,7 +2003,7 @@ psql_completion(char *text, int start, int end)
 			 pg_strcasecmp(prev2_wd, "FROM") == 0)
 	{
 		static const char *const list_DELETE[] =
-		{"USING", "WHERE", "SET", NULL};
+		{"USING", "WHERE", NULL};
 
 		COMPLETE_WITH_LIST(list_DELETE);
 	}