Relax some main thread requirements around waiting for jobs

This is now correctly per-parser so the thread should no longer matter.

src/builtin_wait.cpp

@@ -170,10 +170,7 @@ static bool find_job_by_name(const wchar_t *proc, std::vector<job_id_t> &ids,
     return found;
 }
 
-/// The following function is invoked on the main thread, because the job operation is not thread
-/// safe. It waits for child jobs, not for child processes individually.
 maybe_t<int> builtin_wait(parser_t &parser, io_streams_t &streams, wchar_t **argv) {
-    ASSERT_IS_MAIN_THREAD();
     int retval = STATUS_CMD_OK;
     const wchar_t *cmd = argv[0];
     int argc = builtin_count_args(argv);