Remove `addVsocksHandler` from `loadSnapshotHandlerList`

[CuriousGeorgiy]

According to the firecracker OpenAPI specification ¹, creating vsock
devices is only a pre-boot request, so adding vsocks after loading a
snapshot fails. It also seems redundant, since the VM loaded from a
snapshot restores vsocks anyways.

Footnotes

1. https://github.com/firecracker-microvm/firecracker/blob/56aeeac51c00b449a45be4542b3e807d34690ba7/src/api_server/swagger/firecracker.yaml#L676-L678 ↩