Unblock the package build via static libressl. Adapted from [1].

OpenNTPD requires libtls, which is only available from LibreSSL.
If LibreSSL is not the default SSL library, link with a static copy
of the LibreSSL libraries. This approach has the drawback that
whenever there is an update to libressl, the openntpd port will
require a PORTREVISION bump, otherwise it will not pick up the fixed
library and continue to use the old code.

Drop the RESSL option as LibreSSL is now always available.

PR: 213691 [1]
Submitted by: jbeich