keys,*: eliminate the concept of a SystemConfigSpan

[ajwerner]

Is your feature request related to a problem? Please describe.
Now that #70560 is over and shipped, we can make the system config span a normal key span and rid our codebase of the concept.

Describe the solution you'd like
I'd start by deleting the symbols and seeing what happens. We'll want to change the span config logic which currently deals with it.

Additional context

This is in the way of #81457 because that change wants to add a secondary index to system.users and the index code wants to split off the index. We could definitely just work around that, but it feels good to have a push.

Jira issue: CRDB-16240