[datadogexporter] Remove report_buckets custom error message #5873
Description
Is your feature request related to a problem? Please describe.
In #5858, a custom error message has been added when metrics::report_buckets is set to tell users to use the new config option, metrics::histograms::mode, instead.
Describe the solution you'd like
This custom error can be removed two releases after v0.38.0 (once we're sure most users have stopped trying to use the metrics::report_buckets option). The old option will then still throw a (more generic) error, as it won't be recognised, which is fine.
Describe alternatives you've considered
n/a
Additional context
https://github.com/open-telemetry/opentelemetry-collector-contrib/pull/5858/files#r733832599