Remove cache invalidator feature flag

Part of #1860 (closed)

Closes #2092 (closed)

Since we've seen the cache invalidator perform well in production, it is now time to remove that feature flag so that it is enabled by default for all RPC's.

Originally, this change was supposed to be a simple feature flag removal, but in doing so it caused some unit tests to mysteriously fail. There was an improperly handled edge case with extracting the target repo when it was set to nil. This would return a nil value for the target repo without also returning an error. This caused a nil pointer dereference panic. This change also contains the fix to this issue.