Failing to dispose of webrtc objects before exiting crashes the Editor

[sebjf]

Once created, some webrtc wrapper types must be disposed of before the application quits or else an access violation will be thrown.

One possible cause is that the signalling thread is destroyed before the items are explicitly cleaned, leaving the finaliser to call release, which invokes the destructor, which in turn attempts to run on a destroyed thread.

The desired behaviour is that even when leaking resources, Unity does not crash.

[sebjf]

As a workaround the finaliser in rtc_base.cs was removed. One shouldn't rely on the finaliser anyway.