meta: remove out of date ROADMAP.md file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 <[email protected]>
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: Jeremiah Senkpiel <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>