Exclude MID caches from CRConfig once TM no longer uses CRConfig data

[rawlinp]

This Improvement request (usability, performance, tech debt, etc.) affects these Traffic Control components:

• Traffic Ops
• Traffic Router

Current behavior:

The CRConfig snapshot contains MID caches because Traffic Monitor uses data from the CRConfig currently.

New behavior:

Once #6627 is completed, we should be able to exclude MID caches from the CRConfig, because Traffic Router does not care about MID caches at all. It only needs to know about EDGE caches.