fix(ci): remove unnecessary using directive in CustomDataDirectoryTest #2625

Job	Run time
Build only (iOS)	3m 27s
Build only (Android)	13m 13s
Resource Budget (Source Overrides)	28s
Code Quality	5m 47s
Test (Linux, ubuntu-latest, MultiThreaded, game, osu.Game.Tests/bin/Debug/**/osu.Game.Tests.dll o...	34m 8s
Test (Windows, windows-2025-vs2026, MultiThreaded, rulesets, osu.Game.Rulesets.Osu.Tests/bin/Debu...	13m 39s
Test (Windows, windows-2025-vs2026, MultiThreaded, game, osu.Game.Tests/bin/Debug/**/osu.Game.Tes...	40m 20s
Test (Linux, ubuntu-latest, MultiThreaded, rulesets, osu.Game.Rulesets.Osu.Tests/bin/Debug/**/osu...	10m 47s
Test results	17s
	2h 2m 6s

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