Replacing gtk_button_released() with a button-release-event in commit
13b2f9cd causes crashes, because we probably need to pass an event as
argument to the signal. But we don't have this.

However, it turns out that we don't need that call at all: this was
probably needed for the released signal in ButtonWidget, but on
2004-01-19, it got removed (commit 50855ce2). So the fix is to just
remove this line.