Remove deprecated and unused counters in ENA driver

Few counters were imported from the Linux driver and never used,
because of differences between the Linux and FreeBSD APIs.

Queue stops and resumes are no longer supported by the driver and
counters were incremented indicating false events.

Submitted by: Michal Krawczyk <mk@semihalf.com>
Reviewed by: rlibby
Obtained from: Semihalf
Sponsored by: Amazon, Inc.
Differential Revision: https://reviews.freebsd.org/D12862