Fix: Delete song from BrainzPlayer playlist

[Shreyassp002]

This PR resolves a bug where songs were not being deleted from the playlist in the BrainzPlayer screen. The following changes were made.

• Updated the logic in BrainzPlayerBackDropScreen.kt to properly remove the selected songs (checkedSongs) from the playlist.
• Ensured that the currentPlayable's song list is mutable and modified correctly during the deletion process.

These updates ensure a smoother user experience when managing playlist in the BrainzPlayer feature.

Before:

video_2025-01-01_23-46-14.mp4

Fixed:

video_2025-01-01_23-46-09.mp4

Let me know if there are any changes needed!

[07jasjeet]

Lgtm. We also need to re-think this checked box design-wise because its very out of line wrt to the app.