Remove unused variables for socket AIO.

[jhb]

Summary

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 instead
handles 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.

Test Plan

• Compile.