cli: remove auto-init with cockroach start without --join

[lnhsingh]

PR: cockroachdb/cockroach#51245

From release notes:

A CockroachDB node started with cockroach start without the --join flag no longer automatically initializes the cluster. The cockroach init command is now mandatory. The auto-initialization behavior had been deprecated in version 19.2. [#51245][#51245] {% comment %}doc{% endcomment %}

[jseldess]

@taroface, when you close issues, can you please either link to the PR that resolves it (easiest to do this via the PR itself), or add a note?

[taroface]

Sorry, yes. Closed this one because the 19.2-20.2 docs had already been updated to reflect this change. I honestly can't recall whether I did it or if it was already there.