rndc: prevent crashing after receiving a signal

rndc: prevent crashing after receiving a signal

If isc_app_run() gets interrupted by a signal, the global 'rndc_task'
variable may already be detached from (set to NULL) by the time the
outstanding netmgr callbacks are run.  This triggers an assertion
failure in isc_task_shutdown().  However, explicitly calling
isc_task_shutdown() from rndc code is redundant because it does not use
isc_task_onshutdown() and the task_shutdown() function gets
automatically called anyway when the task manager gets destroyed (after
isc_app_run() returns).  Remove the redundant isc_task_shutdown() calls
to prevent crashes after receiving a signal.