Simplifying QgsOgcUtils

After commit 7d66529d7884b73070dbce80b1d4060d841a6933 Add GML3 to
QgsOgcUtils, I have to simplify this class to only have :
* geometryFromGML : read GML2 or GML3
* geometryToGML : create GML2 (as default) or GML3