overlay: consider removing peer drop based on application-provided predicates

[marta-lokhova]

Consider undoing https://github.com/stellar/stellar-core/pull/4120. A couple of reasons:

• It was originally added as a band-aid, and I'm not certain what the long-term vision for this functionality is. In the meantime, it just adds complexity and maintenance burden to the code with no clear value at this time.
• It's unclear what a meaningful predicate would look like: for example, protocol-based predicates aren't super useful because of core state transitions.

Tagging as "discussion" to get a conversation started.