ci: remove unused step for attesting build provenance #40

Job	Run time
Build and Test (Database.MongoDB, YumeChan.PluginBase.Database.MongoDB/YumeChan.PluginBase.Databa... / build	39s
Build and Test (Base, YumeChan.PluginBase/YumeChan.PluginBase.csproj) / build	32s
Build and Test (Database.Postgres, YumeChan.PluginBase.Database.Postgres/YumeChan.PluginBase.Data... / build	24s
	1m 35s

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