I've added a new toolbar set in config.js file:
['InsertLink', 'Unlink', '-']
Then I went to Article page type, chose a field with Long Text datatype, this datatype has Rich text editor in form control property. And then I filled Toolbar set property with my toolbar set name
After that if I try to create an article, I'll see link buttons only for the field.
Be aware that your browser can save config.js file in cache, so you need to clear it after changes in the file.