Release `CordbUnmanagedThread` instance from `CordbProcess` member (#66910)

* Release CordbUnmanagedThread from CordbProcess member

The m_lastDispatchedIBEvent was not being cleared during shutdown, which was causing a memory leak assert to fire.
Co-authored-by: NAdeel Mujahid <3840695+am11@users.noreply.github.com>