Dropping z dimension programmatically

For future reference - QGIS 3 includes a drop z value processing algorithm

Specifically, the QGIS 3 tool is called Drop m/z values. It's in the Processing Toolbox, under Vector Geometry. The documentation is here:

https://docs.qgis.org/testing/en/docs/user_manual/processing_algs/qgis/vectorgeometry.html#qgisdropmzvalues