IB/rdmavt: No need to cancel RNRNAK retry timer when it is running

When the rdmavt's RNRNAK timer is fired, it tries to cancel the timer by
calling hrtimer_try_to_cancel(), which always returns -1 because the timer
is currently running. This patch removes this useless call.

Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Kaike Wan <kaike.wan@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>