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on the installation medium are inherently out of date, purge them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
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit f61e92ca5a23450bc28169bbdd71d7674df98c19)