Update switch-servers.md: remove bits from CiviCRM 4.7 or earlier, minor cleanup edits

A few minor cleanups:

• Remove mentions of some long-forgotten code
• Minor random edits that caught my eye (this page is very dense, and obviously this MR does not fix everything, but while I was here..)

I was looking at references to the "Rest paths" button, because I think we should remove it from the UI.