[generatepot] Removes unnecessary -b option
[generatepot] Removes unnecessary -b option

To generate pot files using Sphinx, just using "-b gettext"
is sufficient and there is no need to additionally specify
"-b html".

Change-Id: I2eebeb5c575ed99be25980fbd3ec560f3bce2da3

openstack/i18n,openstack/i18n