Remove Quicklook Syntax Highlighting with TextMate2? TextMate2 Alpha comes with a Syntax Highlighting plugin for macOS QuickLook.

How can it be disabled without removing TextMate?
You can run

rm -r /Applications/TextMate.app/Contents/Library/QuickLook/TextMateQL.qlgenerator


but you'll have to do that again every time TextMate is updated.