ssh: add remove-key command to manage SSH keys

[TimPietruskyRunPod]

Problem

runpodctl ssh add-key exists to add SSH keys to an account, and runpodctl ssh list-keys exists to list them, but there is no runpodctl ssh remove-key command to remove a key.

The GraphQL API also has no mutation for removing SSH keys. The only way to remove an SSH key is through the web console, which is not always accessible (e.g. when managing a customer account via API key only).

Expected behavior

Add a runpodctl ssh remove-key command:

# Remove by fingerprint
runpodctl ssh remove-key --fingerprint "SHA256:abc123..."

# Remove by name (if unique)
runpodctl ssh remove-key --name "my-temp-key"

This also requires a corresponding GraphQL mutation (e.g. deletePublicSshKey) to be added to the API.

Workaround

Currently the only option is to log into the web console and manually remove the key from account settings.