Thanks for your answer.
I have found required files and recompiled library. I have commented out g_print lines and no info is showing anymore.
Now to second part of my question - is possible to get motion vector data from my application (using some sort of GstAppSinkCallbacks, GstPadProbeInfo or GstMeta )?
It seems, that function get_motion_vectors used in gstv4l2bufferpool.c do nothing more with founded motion vectors. Is it so?