Can i ask a dumb question becaz i am not sure if i understand about homebridge for home assistant.
By installing this plugin in my Raspberry Pi 2 B where i already have hass 0.40 AiO that this will expose all the devices i have in my hass will be seen on my iPhone Home app and can be controlled by Siri ?
Pretty much, yes.
It’s not really a plugin, it’s a whole other service:
then you’d install the homeassistant module and do some config:
which would make homeassistant controls available via homekit and subsequently home app and siri 
Great i installed this service and my iPhone HomeKit app sees it. Now my question is i how do i remove all the sensors from homekit app?
When i first tried it showed all my devices and i updated the homeassistant config to hide many sensors but that does not remove the devices from homekit app, its just greyed out. So i would like to start over by removing all the devices the homekit found.
remove the bridge from the home app and re-add it