Remove redundant indices on GitLab.com

What does this MR do?

• Remove index_project_mirror_data_on_next_execution_timestamp on project_mirror_data.next_execution_timestamp
• Remove index_push_rules_is_sample on push_rules.is_sample

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/53989, https://gitlab.com/gitlab-org/gitlab-ce/issues/53993

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug