igc: Remove a bogus register write in igc_if_queues_free()

As explained in PR 277038, iflib calls IFDI_DETACH() and then
IFDI_QUEUES_FREE(). With igc, the latter writes to a register after it
has been unmapped.

igc_if_detach() already calls igc_release_hw_control(), and looking at
callers of igc_if_queues_free(), that appears to be sufficient. So,
just remove the igc_release_hw_control() call.

PR: 277038
Reported by: Mike Belanger <mibelanger@qnx.com>
Reviewed by: kbowling
Tested by: kbowling
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D47293

(cherry picked from commit 35d05a14ed7e9935be1ed0fe965b91aaaa4c92ef)