bhyve: Remove the exitcode stats structure

[markj]

Summary

This structure isn't used for anything, and only counts a subset of
vmexit types. Moreover, it is not accurate since there is no
synchronization between vcpu threads. Simply remove it.

No functional change intended.

[jhb]

I suspect the idea was you could examine it via gdb or the like during early development, but it doesn't serve a purpose now.