remove_accelerator
Description:
Removes an accelerator from this, previously installed with add_accelerator.
Parameters:
| this | widget to install an accelerator on | 
| accel_group | accel group for this widget | 
| accel_key | GDK keyval of the accelerator | 
| accel_mods | modifier key combination of the accelerator | 
Returns:
| whether an accelerator was installed and could be removed |