Remove bitbucket_importer_exponential_backoff feature flag

What does this MR do and why?

Removes bitbucket_importer_exponential_backoff feature flag which has been enabled in production since 28 November 2023.

Rollout issue: [Feature flag] Rollout of `bitbucket_importer_e... (#432379 - closed)

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #432379 (closed)