meta: remove out of date ROADMAP.md file

The ROADMAP.md file has not been updated in a while. It serves no
useful purpose that cannot be handled better in other docs
(contrib guidelines, etc). Just remove it.

PR-URL: #11556
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>