agent: remove save/restore migration data code
agent: remove save/restore migration data code

This code is never called, it was relevant for seamless migration.
Most of the data that needs to be migrated was moved to
SpiceCharDeviceState. When we implement seamless migration,
we will have routines in char_device.c for migrating the relevant data.

SPICE/spice,ya-mouse/spice,alon/spice-server,fgouget/spice,elmarco/spice,mathslinux/spice,SPICE/spice,mathslinux/spice,elmarco/spice,elmarco/spice,Fantu/spice,elmarco/spice,freedesktop-unofficial-mirror/spice__spice,alon/spice-server,dezelin/spice,ya-mouse/spice,dezelin/spice,dezelin/spice,fgouget/spice,freedesktop-unofficial-mirror/spice__spice,alon/spice-server,elmarco/spice,ya-mouse/spice,mathslinux/spice,dezelin/spice,fgouget/spice,freedesktop-unofficial-mirror/spice__spice,freedesktop-unofficial-mirror/spice__spice,mathslinux/spice,dezelin/spice,alon/spice-server,Fantu/spice,Fantu/spice,freedesktop-unofficial-mirror/spice__spice,Fantu/spice,mathslinux/spice,SPICE/spice,fgouget/spice,alon/spice-server