Remove some legacy declarations from <math.h>.

These are still needed for backwards compatibility with code built by old
versions of the NDK, but we don't need to pollute the headers with them.

Also lose the hand-written code for these. The compiler-generated code
is either the same or better, and no new code is calling these functions
anyway.

Bug: N/A
Test: ran tests
Change-Id: Ib01ad9805034433e0105aec882608cc8e6526f78