Drop M values from MULTIPOLYGON with PostGIS?

The function is ST_Force3D if you have XYZM geometry, if it's XYM you should use ST_Force_2D.

From the manual:

• ST_Force_2D — Forces the geometries into a "2-dimensional mode" so that all output representations will only have the X and Y coordinates.
• ST_Force_3D — Forces the geometries into XYZ mode. This is an alias for ST_Force_3DZ.
• ST_Force_3DZ — Forces the geometries into XYZ mode. This is a synonym for ST_Force_3D.
• ST_Force_3DM — Forces the geometries into XYM mode.
• ST_Force_4D — Forces the geometries into XYZM mode.