Remove bvmconsole and bvmdebug.

Now that bhyve(8) supports UART, bvmconsole and bvmdebug are no longer needed.

This also removes the '-b' and '-g' flag from bhyve(8). These two flags were
marked deprecated in r368519.

Reviewed by: grehan, kevans
Approved by: kevans (mentor)
Differential Revision: https://reviews.freebsd.org/D27490