bhyve: Remove handler for VM_EXITCODE_SPINUP_AP.

Since commit 0bda8d3e9f7a, bhyve always enables VM_EXITCODE_IPI exits
instead, so this handler is no longer used.

Reviewed by: corvink, markj
Differential Revision: https://reviews.freebsd.org/D37640

(cherry picked from commit 007d9ca5dd953c009cf976b49e2568444a16a473)