This device was showing up from a LG 38UC99-W USB-C monitor
```
VMM0000:
Device ID: d762543f8c20f636e6fff031a000078d3e10c600
Summary: Multi-Stream Transport Device
Current version: 0.00.000
Vendor: Synaptics
GUIDs: 42addef4-40f9-5e89-b925-d564e35ed368 ← MST-(null)-vmm0000-0
cf8c03c5-18bf-53c4-971f-4a08f88932b5 ← MST-(null)-0
e9427b6a-7389-5461-a592-1da5f8ec99fd ← MST-(null)
Device Flags: • Updatable
```
This removes the 'two-layer' FuDevice and FuSyanpticsmstDevice model, where
a complicated plugin cache was used to add and remove devices to the daemon.
By making FuSynapticsmstDevice derive from FuDevice rather than GObject we can
also use a lot of the helper functionality like the other plugins, for instance
->prepare_firmware().
The `drm_dp_aux_dev` devices do not emit uevents on unplug/re-plug and so all
devices of `drm` class are watched and the actual DP AUX devices rescanned after
a small delay. When the AUX devices emit changes from the kernel this workaround
can be removed.
Also drop force power support for MST controllers in the Dell plugin. Overall
this has just led to more problems than it's helped.
* Monitor flickers when turned on
* Crashing graphics drivers from time to time
* Fragile logic that doesn't always represent the device state
