bluetooth: complete removal of ng_h4

The ng_h4 module was disconnected 13 years ago when the tty later was
locked by Ed. It completely fails to compile, and has a number of false
positives for Giant use. Remove it for lack of interest. Bluetooth has
largely (completely?) moved on from bluetooth over UART transport.

OK'd by: emax
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D31846