i40e: remove ptp_tx_work timestamp work item

This patch removes the unnecessary ptp_tx_work item. It conflicts with
the interrupt-based handler, and will cause spurious warning messages to
be displayed in the kernel log. Since the hardware can properly trigger
an interrupt, we do not need to poll for an available timestamp in a
work queue any more.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Acked-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>