Remove unused CmMathUtils.cpp from the PhysX copy

This file is not part of upstream PhysX 4.1.2 at all: it is a
byte-identical leftover from 4.1.1, hence its 2008-2019 copyright,
the only such line in the tree. It has been excluded from the build
since "Various fixes for passing CI", and is the only .cpp here not
referenced by CMakeLists.txt.

The guard against it in find_includes.py is kept, so a future import
of a PhysX version that does contain the file will not silently add it
back to the build.

Pick-to: 6.12
Change-Id: I2ca8e26fafd7b73eb779694769b3de7d028648d6
Reviewed-by: Jonas Karlsson <jonas.karlsson@qt.io>