Remove ifdef BOOTCDROM option to start init.

When BOOTCDROM is defined (via CFLAGS as there is no config option)
it causes -C to be passed to init, but our init and the version of
sysinstall I glanced at in 6.x don't support -C. The last plausibly
related support was removed from the tree in 1995.

Reviewed by: kib
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D18431