Removes storage of `selectedBeatmap` that was referenced through
multiple class-level methods.
To expound a bit, this structure felt better (or otherwise passing
`APIBeatmap` through methods) alongside removal of the `#nullable
disable`, otherwise each method would check `selectedBeatmap != null`.
Though the code appears slightly different, it should be semantically
equivalent. APIUser equality is implemented on `Id` and `Host` should
never transition from non-null to null.