synapticsmst: Use fu_plugin_device_removed signal
synapticsmst: Use fu_plugin_device_removed signal

If another plugin causes a Synaptics MST device to be removed, it
also needs to be removed from the cache to prevent problems when
the device is re-probed.

vathpela/fwupd,hughsie/fwupd,fwupd/fwupd,fwupd/fwupd,hughsie/fwupd,vathpela/fwupd,hughsie/fwupd,hughsie/fwupd,fwupd/fwupd,vathpela/fwupd,vathpela/fwupd,fwupd/fwupd