Do not delete a ShellCmd remote job's PID file until

`ShellcmdLrms.free()` is called.

Removal of remote job's PID file should only be done in the
`ShellcmdLrms.free()` method, and *not* at other stages of the job's
lifecycle.

gc3libs/backends/shellcmd.py

@@ -1091,7 +1091,6 @@ def cancel_job(self, app):
             else:
                 # ignore
                 pass
-        self._delete_job_info_file(root_pid)
 
     def _grace_time(self, wait):
         if wait:
@@ -1656,8 +1655,6 @@ def _cleanup_terminating_task(self, app, pid, termstatus=None):
                    .format(wrapper_filename, app, err))
             log.warning("%s -- Termination status and resource utilization fields will not be set.", msg)
             raise gc3libs.exceptions.InvalidValue(msg)
-        finally:
-            self._delete_job_info_file(pid)
 
     def _parse_wrapper_output(self, wrapper_file):
         """