protocols/gossipsub: Remove regex-filter feature flag (#2056)

Remove `regex-filter` feature flag thus always enabling
`regex::RegexSubscriptionFilter`.

protocols/gossipsub/src/subscription_filter.rs

@@ -200,7 +200,6 @@ where
     }
 }
 
-#[cfg(feature = "regex-filter")]
 pub mod regex {
     use super::TopicSubscriptionFilter;
     use crate::TopicHash;