Replace Rake's Win32-specific logic with a 100% equivalent, pure-Ruby implementation

[pvdb]

tl;dr - remove some win32-specific Rake logic that has long since been implemented by Ruby itself

Rake's custom Windows-specific implementation (Rake::Win32::win32_system_dir) produces identical results to Ruby's standard library approach (File.join(Dir.home, "Rake")) for all possible scenarios it caters for, so this PR removes a whole bunch of superfluous code and corresponding tests, and replaces it with a much simpler, pure stdlib-based implementation.

Because it's hard to use and test something that isn't there anymore :smile: I've raised an accompanying DRAFT pull request #670 that explicitly tests the equivalency of both versions against the master branch (for a variety of different scenarios) thereby demonstrating that the Rake-specific patch is superfluous and can be safely removed.