Remove obsolete codeSuggestionsUsageRate field
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)