BButton: Removed dead code

Remove methods _DrawDefault() and _DrawFocusLine() which were used for
the non-BControlLook drawing.
BControlLook::GetFrameInsets(): Fix button insets

The button frame is 3 pixels wide, but that includes one pixel that
actually belongs to the background, which is accounted for by
GetBackgroundInsets().
Removes the additional one pixel inset accidentally introduced earlier.