Switch `SampleStore` `ConcurrentDictionary` usage to be a normal `Dictionary`
Switch `SampleStore` `ConcurrentDictionary` usage to be a normal `Dictionary`

Thread safety is already enforced via a `lock`, so this is unnecessary.

ppy/osu-framework,ppy/osu-framework,smoogipooo/osu-framework,peppy/osu-framework,peppy/osu-framework,peppy/osu-framework,smoogipooo/osu-framework,ppy/osu-framework