Remove custom ref resolver passed to JSONSchemer

What does this MR do and why?

This MR removes the custom definition reference resolver which is passed to JSONSchemer.

JSONSchemer is already defining a ref_resolver to resolve local files for us in case a Pathname is passed (which is always the case in specs).

See #351777 (comment 1244193128)

Follow-up of !108588 (closed) where ref_resolver was removed and 🔴 was expected but it was 💚 🤷

How to set up and validate locally

# Run metric dashboard related specs
bin/rspec $(rg 'valid dashboard service response' -l) -e "valid dashboard service response"

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