Remove obsolete codeSuggestionsUsageRate field

Pavel Shutsinrequested to merge
456070-cleanup-obsolete-api into master

What does this MR do and why?

Removes obsolete codeSuggestionsUsageRate field

Please use codeSuggestionsContributorsCount and codeContributorsCount to calculate usage rate

No user facing changes here. It was confirmed that the field is not in use anymore.

API is still Alpha so we can freely change it even with breaking change

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Related to #456070 (closed)

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