Remove message manager lock assert from AudioThumbnail::setDataSource

[kf6kjg]

Digging though the commit history didn't show any reason why this should exist. Reading the code didn't readily indicate any reason it should exist. It's existence meant I couldn't cleanly make my thumbnails in a separate thread. So I've been running with the scissors and have removed it from my own copy.

Forum post located at https://forum.juce.com/t/why-does-audiothumbnail-setdatasource-need-the-message-manager-locked/34023