bhyve: Do not define GDB_LOG

This had been added for debugging and shouldn't have been committed.

Approved by: re (cperciva)
Fixes: f81cdf24ba54 ("bhyve: Add support for XML register definitions")
MFC after: 3 days

(cherry picked from commit 5d62025d82a0be928f98778d54b25ad89edbb835)
(cherry picked from commit d8ccaa995f4539081f79e9b31ff0ac75173e1004)