Drop bulk_import_trackers sharding key triggers

What does this MR do and why?

Drop bulk_import_trackers sharding key triggers

First introduce in !180437 (merged), I'm removing these triggers because they were too permissive in that they could set more than one column in a multi-column sharding key. That's a problem because we want to limit multi-column sharding keys to exactly one (1) column. See this comment: !188587 (comment 2651635058).

Addresses #560846

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