Drop partitions_dynamic schema if it exists

What does this MR do?

!34504 (comment 368919595) introduced the partitions_dynamic schema. The code change got reverted later, but it was deployed to gstg/gprd. That is, those environments now have a partitions_dynamic schema, but should not.

If we don't have a means to rollback this migration on said environments, this MR comes in to add another migration ("rollforward") to drop the schema if it exists.

!35137 (merged) finally introduced gitlab_partitions_dynamic schema, which is the desired schema to keep.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content