flux: drop unnecessary timeout from Flux plugin

Problem: The 300s timeout in the spindle flux plugin may cause large
jobs experiencing slow startup to fail unnecessarily.

Drop this timeout as unnecessary since the `shell.init` event is
guaranteed to come at some point or the job will be timed out in some
other manner.

Author: grondo

src/flux/flux-spindle.c

@@ -649,7 +649,7 @@ static int sp_init (flux_plugin_t *p,
      *   rank 0, but code is simpler if we treat all ranks the same.
      */
     if (!(f = flux_job_event_watch (h, id, "guest.exec.eventlog", 0))
-        || flux_future_then (f, 300., wait_for_shell_init, ctx) < 0)
+        || flux_future_then (f, -1., wait_for_shell_init, ctx) < 0)
         shell_die (1, "flux_job_event_watch");
 
     /*  Return control to job shell */