Remove math.h/cmath include from DataTypes.h

DataTypes.h is meant to wrap the integer type and limits headers, which
have some unfortunate variance. The FP math functions declared by math.h
are not unnecessary. math.h took a noticeable amount of time to parse
(~40ms), but that could be startup costs.

Anyway, we don't need to include it, so skipping it can't hurt.

This has been present since the initial CMake build was added in 2008.