Remove redundant same-shard check for cross-shard updates

[sgrif]

We don't need to check this in insert_row, we already checked if we're on the same shard in UpdateMulti::execute_internal before calling this

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
.../frontend/client/query_engine/multi_step/update.rs	83.33%	4 Missing ⚠️

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