Check if RawContentManager still makes sense after MonoGame fully introduces raw content loading

[Ellpeck]

See https://monogame.net/blog/2024-09-11-meeting/#content-pipeline-enhancements.

Things to consider:

• Does the regular content pipeline even allow loading assets from outside the game's source (ie without TitleContainer)? We want to still be able to with RawContentManager for moddability purposes
• Is there any noticeable performance benefit to our implementation (which doesn't check for existing xnb files first)?
• Would it make sense to delegate RawContentManager's MG impl to the base class, but still keep RawContentManager for other platforms like FNA and KNI?