frontend: minor SPICE cleanup

This patch removes confusing code that acts like it's responsible for
installing SPICE plugin. In 3.y era we actually don't support this (on
windows, the plugin installation is handled internally by activex, on
Linux we don't support plugin installation at all.).

Change-Id: Ib3af00d6eba93756db881229edd112085f217260
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>