i40e: remove debugfs tx_timeout support

The tx_timeout command from debugfs was originally intended to support
early silicon validation efforts. It is no longer needed. Thus remove it to
avoid misuse of triggering tx_timeout through debugfs.

Signed-off-by: Lihong Yang <lihong.yang@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>