Windows: Cleanup header includes and definiions

Windows: Cleanup header includes and definiions

Now that a sufficiently recent toolchain is required to build, we can
drop a bunch of redundant definitions and get the definitions from the
Windows headers instead. We can also remove a number of '#ifndef'
guards because no header included by the source would have defined the
symbols being protected.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>