When I installed komodo it became a default editor for text files, markdown and etc. In the Preferences I found File Association option, but it seems that it has other purpose (or do I need to manually remove every association?). I just don’t want to open any files with komodo and want gedit back as my default editor.
It’s a Komodo preference, not an OS preference. These settings only gives some info how Komodo should do syntax colorizing depends on the file extension. Like if you want to see Markdown syntax colorizing for each *.my-markdown file you should add a new item to the file association preferences.
Komodo does not set itself as the default, that’s all handled on the OS level without any interference from Komodo. So you’d have to look for documentation on how to change default apps for the OS that you are using. This looks promising.