Remove `$EASYRSA_LEGACY_SAFE_SSL` and relevant code

[TinCanTech]

Global option --old-safe-ssl is required to trigger use of $EASYRSA_LEGACY_SAFE_SSL

This also removes use of sed, to expand openssl-easyrsa.cnf

This is postponed until v3.2.1, to allow this long standing method to be included in at least one v3.2.x release.