keygen: remove redundant error handler
    
    Fixing the following pylint warning:
    
        E0102[function-redefined]: handle_invalid_usage:
        function already defined line 132
    
    We could rename one of the error handler functions, but we can IMHO
    drop one of them and simply put multiple `@app.errorhandler`
    decorators to one function.
    
    Merges: #2251