Multisite Network Setup page still renders after multisite has been set up

[jazzsequence]

I believe this is the result of adding in our own modified version of the Network Setup page.

After you've set up your multisite, the Network Setup page still exists under the Tools menu. Our documentation isn't super clear that it's not intended to be read from top to bottom (rather than as a choose your own adventure) (see pantheon-systems/documentation#9322), so if you've already set up the multisite via terminus and added the constants, you don't actually need that network setup page at all.

In the case that you added the constants to your wp-config already, that page is just blank. We should either:

• remove the page from the Tools menu if the constants exist
• keep the page but display a message that says something to the effect of "your multisite is already configured" (maybe we could output the multisite constants or something, I dunno)