Fix loading of all sensorgesture plugins.

After the change in qtbase 732dcfe7, only the first
sensorgesture plugin was actually being loaded.
Since the FILE argument of Q_PLUGIN_METADATA is optional,
removing that and the json file fixes the following bugs:

QTBUG-31900
QTBUG-31901

Change-Id: I7c0b67605a8fe680c37639f08926140e137dbe66
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>