The file /boot/boot.conf existed for the 3.0 release (r38764). It was
replaced by /boot/loader.rc for 3.1 (r42682). In May 2000, this was
documented as deprecated (r61942) (between FreeBSD 4.0 and
4.1). Remove it since it's not been the preferred method in 17 years
and has been deprecated for 16.

Differential Revision: https://reviews.freebsd.org/D8142