release: permanently remove the 'reldoc' target and associates

release: permanently remove the 'reldoc' target and associates

Following 7b1d1a1658ffb69eff93afc713f9e88ed8b20eac, the structure
for the reldoc target has significantly changed as result of the
ASCIIDoctor/Hugo migration.  As the release notes related files
on the installation medium are inherently out of date, purge them
entirely.

Discussed within:	re, doceng
No objection:		re (silence), doceng (silence)
Timeout:		2 weeks
MFC after:		1 week
MFC to:			stable/13, stable/12, and stable/11 only
Sponsored by:		Rubicon Communications, LLC ("Netgate")