Remove unnecessary `deepcopy` call #3827

Job	Run time
MetricFlow Tests (PostgreSQL)	3m 51s
MetricFlow Packages Tests (3.8)	3m 28s
MetricFlow Tests (DuckDB) (3.8)	3m 9s
MetricFlow Tests (DuckDB) (3.12)	2m 44s
MetricFlow Packages Tests (3.12)	3m 31s
MetricFlow Unit Tests	1s
	16m 44s

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