Remove unused pytest warning filters #776

Job	Run time
Formatting and static analysis	12s
Mantid tests	1m 31s
Tests (ubuntu-24.04, 3.11, py311) / test	2m 54s
Tests (ubuntu-24.04, 3.11, py311) / package-test	21s
docs / Build documentation	8m 28s
	13m 26s

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