Remove an accelerator (keyboard shortcut).
The modifiers is a bitwise or of one or more of GDK.ShiftMask,
GDK.LockMask, GDK.ControlMask, GDK_MOD1_MASK , GDK_MOD2_MASK , GDK_MOD3_MASK , GDK_MOD4_MASK and GDK_MOD5_MASK .
The group is the accelerator group in which the accelerator should be added.
The key is the unicode code for the key to bind.