Remove 'ixgb' from HEAD

[Kevin Bowling]

The ixgb driver was used only briefly by one sub family of devices in
the first half of the 2000s.  It's not a power efficient card with a
massive on board xenpak.  The driver saw little specific (not
API/treewide stuff) maintenance post FreeBSD 5 in which the last real
intentional commit directly on the driver was by someone admitting
they didn't have a card but spotting a bunch of errors in the driver.
It's unlikely the driver is suitable for commercial use as it stands,
and I wouldn't trust it in a home server.

If someone actually uses ixgb and sends me one I will flatten it into
iflib ixgbe.  You can get one for $250 here :o)
https://www.ebay.com/itm/Intel-PRO-10GbE-SR-PCI-X-PXLA8591SR-Server-Adapter-/110895948350

Otherwise I propose it be removed in HEAD, which gives a generous
deorbiting period of September 30, 2021 in stable/11.

Regards,
Kevin