libcamera: fix a use-after-free due an attempt to stop the camera twice

Camera is currently stopped and started in the spa_libcamera_set_format()
function, but this leads to a segfault due attempting to access a buffers
field of an already freed libcamera::FrameBufferAllocator instance.

The FrameBufferAllocator instance is freed in LibCamera::stop(), that is
called by spa_libcamera_stream_off() as handler of the node commands
SPA_NODE_COMMAND_Pause and SPA_NODE_COMMAND_Suspend.

Since the camera was already stopped, there's no need to attempt to stop
it again. In fact, the camera shouldn't be stopped/started at all in the
spa_libcamera_set_format() function but instead only as an action of the
SPA_NODE_COMMAND_{Pause,Suspend} and SPA_NODE_COMMAND_Start commands.

And same for the stop that's done in the LibCamera::close() function, it
shouldn't be needed because the camera is already stopped before closing.

Fixes #1513