xen-netfront: remove XN_LOCK_{INIT,DESTROY}

Multiqueue feature will make the number of queues dynamic, so XN_LOCK_INIT
won't be that useful. Remove the macro and call mtx_init directly.

XN_LOCK_DESTROY is just dead code.

Submitted by: Wei Liu <wei.liu2@citrix.com>
Reviewed by: royger
Differential Revision: https://reviews.freebsd.org/D3890
Sponsored by: Citrix Systems R&D