Remove manual wire_count adjustments from the unmapped mbuf code.

The original code came from a desire to minimize the number of updates
to v_wire_count, which prior to r329187 was updated using atomics.
However, there is no significant benefit to batching today, so simply
allocate pages using VM_ALLOC_WIRED and rely on system accounting.

Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D21323