QMetaObjectPublisher: Remove the transport state with the transport

transportState is a QHash keyed by the raw transport pointer. It holds
the "client is idle" flag and the outgoing message queue for each
transport. transportRemoved() did not erase the entry, thus the entry
outlived the transport that it describes.

Fixes: QTBUG-150200
Fixes: QTBUG-151078
Pick-to: 6.8 6.12
Change-Id: I583475524da16354bd072a47321ea8f76092d572
Reviewed-by: Magdalena Stojek <magdalena.stojek@qt.io>