[vms/avm] Remove all keystore APIs except `ExportKey` and `ListAddresses`

[dhrubabasu]

Why this should be merged

These functions should not be used as keys should not be stored on the node. We'll retain ExportKey and ListAddresses to enable people to export their keys if they have not done so already.

How this works

Deletes all code relating to keystore APIs in the avm except ExportKey and ListAddresses.

How this was tested

• [x] CI

[github-actions[bot]]

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