[ntcore] Remove table multi-subscriber (#4789)

This wasn't well thought out, and leaks horribly in Java.

Reverts part of #4640.

ntcore/src/main/native/include/networktables/NetworkTable.h

@@ -48,7 +48,6 @@ class NetworkTable final {
  private:
   NT_Inst m_inst;
   std::string m_path;
-  NT_MultiSubscriber m_topicSub;
   mutable wpi::mutex m_mutex;
   mutable wpi::StringMap<NT_Entry> m_entries;