Consolidate PeerConnection network thread teardown

This patch introduces a new private method, `UninitializeNetworkThread`,
to deduplicate the logic responsible for destroying the transport
controller and port allocator on the network thread. Previously, this
teardown logic existed in both the `~PeerConnection` destructor and the
`PeerConnection::Close` method.

Refactoring this into a single method ensures consistent behavior for
thread safety checks, candidate pool discarding, and pointer resetting.

In addition, this change:
* Updates `DestroyDataChannelTransport` to return early if the data
    channel is not configured, avoiding unnecessary context switching.
* Removes the rvalue reference qualifier from the `FakePortAllocator`
    callback `on_candidate_pool_discarded_`, simplifying its invocation.

Bug: webrtc:360058654
Change-Id: Iac66c2469c8b2180b88d6fe583ce8fcd07d16ff5
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/427360
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46274}