Remove the server parameter introduced in SERVER-38594

Additionally, RollbackImpl::_resetDropPendingState() will no longer have to call DropPendingCollectionReaper::clearDropPendingState() because drop-pending idents are assumed to be supported when this rollback algorithm is used (the alternative is the rollback via refetch implementation in rs_rollback.cpp).

With the changes in SERVER-38527, we can remove supportsPendingDrops() from the storage engine API layer and use supportsRecoveryTimestamp() instead. Additionally, the kv_storage_engine IDL can be removed as it was only used for "enableKVPendingDrops".