Deprecate and remove the shutdown command

During this week's triage of https://github.com/ipfs/go-ipfs/issues/7865 we realized that `shutdown` command exposed over HTTP API is pretty risky thing to have.

IIUC go-ipfs already supports shutdown signals the right way:

https://github.com/ipfs/go-ipfs/blob/ea77213e31ef2b3cad81d40bf82bb9baef3ea7b6/cmd/ipfs/util/signal.go#L79

I propose deprecation and removal of the `shutdown` command.
Either someone stops `ipfs daemon` via ^C or send a proper signal to the process:
No need of having third way which is exposed via HTTP API.

cc @aschmahmann @Stebalien @whyrusleeping thoughts? is there any reason to keep it?

Start by reviewing the shutdown command and where it is exposed through the HTTP API, then compare that behavior with cmd/ipfs/util/signal.go at the referenced signal handling code. Confirm the deprecation and removal scope with maintainers, and consider the work done when shutdown is no longer exposed and signal-based termination remains supported.

Written by the indexing model from the issue text.