Remove the UpdateMicroVM Endpoint

[richardcase]

Describe the solution you'd like:
As update a microvm isn't needed for our initial use case (i.e. capi) and as Firecracker microvms can only be updated in a few limited ways lets remove the UpdateMicroVM rpc endpoint and any associated code.

Why do you want this feature:

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]