libcamera: CameraManager: Drop the vector of created PipelineHandlers

The pipes_ vector was initially used to store pipeline handlers
instances with the CameraManager when it cannot be referenced from
anywhere else. It was used to retrieve cameras and deleting pipeline
handlers when stopping the camera manager.

In f3695e9b09ce ("libcamera: camera_manager: Register cameras with the
camera manager"), cameras started to get registered directly with camera
manager and in 5b02e03199b7 ("libcamera: camera: Associate cameras with
their pipeline handler") pipeline handlers started to get stored in a
std::shared_ptr<> with each camera starting to hold a strong reference
to its associated pipeline-handler. At this point, both the camera
manager and the camera held a strong reference to the pipeline handler.

Since the additional reference held by the camera manager gets released
only on cleanup(), this lurking reference held on pipeline handler did
not allow it to get destroyed even when cameras instances have been
destroyed. This situation of having a pipeline handler instance around
without having a camera may lead to problems (one of them explained
below) especially when the camera manager is still running.

It was noticed that, there was a dangling driver directory issue (tested
for UVC camera - in /sys/bus/usb/drivers/uvcvideo) on 'unbind' → 'bind'
operation while the CameraManager is running. The directories were still
kept around even after 'unbind' because of the lurking reference of
pipeline handler holding onto them. That reference would clear if and
only if the CameraManager is stopped and then only directories were
getting removed in the above stated path.

Rather than writing a fix to release the pipeline handlers' reference
from camera manager on camera disconnection, it is decided to eliminate
the pipes_ vector from CameraManager moving forwards. There is no
point in holding a reference to it from camera manager's point-of-view
at this stage. It also helps us to fix the issue as explained above.

Now that the pipeline handler instances are referenced via cameras only,
it can happen that the destruction of last the camera instance may
result in destruction of the pipeline handler itself. Such a possibility
exists in PipelineHandler::disconnect(), where the pipeline handler
itself can get destroyed while removing the camera. This is acceptable
as long as we make sure that there is no access of pipeline handler's
members later on in the code path. Address this situation and also add a
detailed comment about it.

Signed-off-by: Umang Jain <email@uajain.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

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