Remove ServerCallbackIface

It's currently unused

Change-Id: I8abd275f127b17ce9beeea1b984c78c7d44047c1
Reviewed-on: https://review.couchbase.org/c/kv_engine/+/179283
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Richard de Mellow <richard.demellow@couchbase.com>

include/memcached/engine.h

@@ -82,14 +82,12 @@ class StatCollector;
 struct DocKey;
 struct ServerBucketIface;
 struct ServerCoreIface;
-struct ServerCallbackIface;
 struct ServerCookieIface;
 struct ServerDocumentIface;
 union protocol_binary_request_header;
 
 struct ServerApi {
     ServerCoreIface* core = nullptr;
-    ServerCallbackIface* callback = nullptr;
     ServerCookieIface* cookie = nullptr;
     ServerDocumentIface* document = nullptr;
     ServerBucketIface* bucket = nullptr;