Do not install the dummy sensors plugin by default

The dummy sensors plugin will start fake sensors that emit 100 updates
per second. This should not be the default sensor returned when a
user requests a sensor.

These sensors were default on Linux.

Task-number: QTBUG-39475
Change-Id: Ia276e904f93dfeef95f7490ec7cb51cad260a2a8
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>