Revert "Bravely set job control to full at startup"

Now that `$last_pid` is never fish's pid, we no longer need to force
jobs to run in their own pgroup. Restore the job control behavior to
what it was prior, so that signals may be delivered properly in
non-interactive mode.

This reverts commit 3255999794

src/fish.cpp

@@ -497,9 +497,6 @@ int main(int argc, char **argv) {
     misc_init();
     reader_init();
 
-    // And now enable "job control" for everything.
-    set_job_control_mode(job_control_t::all);
-
     parser_t &parser = parser_t::principal_parser();
 
     if (!opts.no_exec && !opts.no_config) {