add ssh remove-key command

[vavo]

E-258: add ssh remove-key command

fixes #258

summary

• add runpodctl ssh remove-key with --fingerprint and --name
• remove keys by rewriting pubKey through the existing updateUserSettings graphql path
• return an ambiguity error when --name matches multiple keys
• add unit coverage for command wiring and graphql removal behavior
• regenerate ssh command docs

test plan

• go test ./...
• live e2e against runpod api
  blocked locally: RUNPOD_API_KEY was not available in this workspace, so i could not run the required live ssh add/list/remove flow

[vavo]

superseded by #264, which uses the requested branch name notrius/issue-258-ssh-remove-key and the repo pull request template.

[vavo]

closing this duplicate draft in favor of #264