Theme/Plugin Editor: Remove the caching added in [41806] as it causes more problems than it fixes.

While caching here seemed like a good idea in theory, in practice the cache would be often stale causing development issues.
We exclude common folders (such as `node_modules`) from the scanning to avoid directories which are not useful to the end-user, so as long as those exclusion lists are held up this shouldn't cause too much of a degredation in the future.
We may consider adding caching here again in the future if it's determined that it is really needed.

Props precies, ibenic, mariovalney, schlessera, and all the others who commented on the ticket(s).
This partually reverts [41806].
See #6531.
Fixes #42573.

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git-svn-id: https://core.svn.wordpress.org/trunk@42071 1a063a9b-81f0-0310-95a4-ce76da25c4cd