[DOC] Removed note about SAHARA_USE_NEUTRON in sahara-dashboard

Sahara-dashboard was merged to horizon. Horizon detects neutron by
itself and no additional settings are needed. Deleted the whole
section.

Change-Id: Id6bca92f924d271eed5a78b912b4014e25bb1ea8
Merge "[DOC] Removed note about SAHARA_USE_NEUTRON in sahara-dashboard"