staging: rtl8188eu: remove RegulatorMode from struct hal_data_8188e

This field is not used anywhere.  Also, kill the rt_regulator_mode enum
which exists just for this field.

Signed-off-by: Jakub Sitnicki <jsitnicki@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>