We are sorry for the troubles!
When enabling TK2D support accidentally and closing the Unity Editor, the cat bites it's tail, as the Preferences menu is gone when closing Unity and only reappears when no compile errors are left.
A solution is to manually remove the SPINE_TK2D defines from the ProjectSettings file:
ProjectSettings/ProjectSettings.asset
and perhaps also from
Library/ProjectSettings.asset
Just open it with a text editor and edit it accordingly.
The relevant section looks like this:
scriptingDefineSymbols:
1: SPINE_TK2D
