Drop prometheus_initializer_refactor feature flag

What does this MR do and why?

This removes the FF prometheus_initializer_refactor. It was enabled by default and has been live for a while now and we haven't seen any regressions.

Note that the changes here look noisier than they are, because the new code path was in a separate file, 7_prometheus_new.rb, which I have now renamed to the original 7_prometheus.rb, so all of the diffs from the original MR show up again.

All I changed though was to remove the early return that performs the FF check and rename that file back.

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Related to #353446 (closed)