Remove red wavy underline from CheckStyle logo

[l0kix2]

In IDEA it looks like this

and it looks like «always errors» and it is annoying. Maybe we can leave only letters on logo and remove underline?

[jshiell]

If you've got errors in your tool windows then you've bigger problems 😂

I've changed it to use the small Checkstyle icon (the little pencil) instead, which will probably play more nicely with the dark themes in any case.

[l0kix2]

thanks)