MK_FORTH: Allow removing loader.4th w/ delete-old

The lua loader is quite mature, and /boot is getting a bit cluttered.
Allow removing the Forth loader with make delete-old if WITHOUT_FORTH
is set, instead of the previous behavior of only removing the doc.

MFC: no, I think this is a breaking change
Approved by: kevans (src)
Reviewed by: kevans (UPDATING note tweaked since)
Discussed with: imp, tsoome (thanks!)
Differential Revision: https://reviews.freebsd.org/D50708