Remove unused variables for socket AIO.

In r55943, a per-process queue of pending socket AIO requests (requests
waiting for the socket to become ready) was added so that they could be
cancelled during process rundown. In r154765, the rundown code was
changed to handle jobs in this state (JOBST_JOBQSOCK) directly removing
the need for the extra queue. However, the per-process queue head and
global lock were never removed.

Reviewed by: kib
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D4997