[libc++] Remove math_win32.h

Visual Studio 2013 and onward have all the required functions in their
CRT headers, and we don't support older versions anymore.

Differential Revision: https://reviews.llvm.org/D24879

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@282328 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed