Somewhat clean up win64.mak

[kinke]

• Remove some unused variables.
• Default to a matching MSVC cl.exe & lib.exe in PATH instead of an ancient Visual Studio 2010 installation.
• Remove explicit 32-bit recipes, requiring a separate make invocation.

Analogous to dlang/druntime#3853.

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[kinke]

Looks like this is unused - it a) didn't make sure the lib is actually built beforehand, and b) tried to copy a phobos.lib, which should have never existed (no MODEL suffix).