Remove some redundant code for bool opt actions

You can see how this code came about here:

  https://review.openstack.org/#/c/14125/20..21/openstack/common/cfg.py

but the only time BoolOpt:_get_argparse_kwargs() gets passed the action
kwarg is from _add_inverse_to_argparse() where we pass 'store_false'.

Change-Id: Ic6aa123680b5eb9aa220152389f3535b703fdf7b