Remove manyToMany warnings #1373

Job	Run time
@dataplan/pg tests / build (12, 20.x)	3m 45s
PostGraphile mutation tests / build (12, 20.x)	3m 24s
PostGraphile mutation tests (exported as graphql-js) / build (12, 20.x)	10m 18s
PostGraphile mutation tests (exported as typeDefs) / build (12, 20.x)	8m 33s
PostGraphile polymorphic query tests / build (12, 20.x)	3m 57s
PostGraphile polymorphic query tests (exported as graphql-js) / build (12, 20.x)	8m 2s
PostGraphile polymorphic query tests (exported as typeDefs) / build (12, 20.x)	7m 15s
PostGraphile v4 queries tests / build (12, 20.x)	4m 46s
PostGraphile v4 queries tests (exported as graphql-js) / build (12, 20.x)	18m 3s
PostGraphile v4 queries tests (exported as typeDefs) / build (12, 20.x)	13m 48s
PostGraphile schema tests / build (12, 20.x)	6m 44s
Small package tests / build (12, 20.x)	4m 29s
	1h 33m 4s

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