Remove hard-coded Windows installation paths

- For Git, shouldn't need to look in Program Files since Windows Julia bundles Git.
- MinGW is a build-time dependency, not a user dependency. These path additions were the only reason test/spawn.jl could ever execute successfully from a Windows command prompt (see for example https://github.com/JuliaLang/julia/issues/3548#issuecomment-36280804). This is a problem that can be addressed separately, and the solution should not rely on a hard-coded absolute path to MSYS1 (unless we start bundling our own MSYS in the Julia installer, in which case we'd use a relative path).
- Python should only be needed by packages, or at build time for LLVM. Wouldn't a better solution involve PYTHONPATH or PYTHONHOME, or manage this on a package level when Python is required?