tl;dr - replace some Win32-specific Rake logic for Windows home directory detection logic with Ruby's built-in Dir.home method

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.

It is replaced with a much simpler, pure stdlib-based implementation. in a low-risk manner: Ruby's cross-platform Dir.home has been available since Ruby 1.9 and is the standard way to get the home directory; this change simply stops reinventing the wheel. 😃

1. Simpler code — Removes ~85 lines of custom Win32 path handling code, tests, and a custom exception class
2. More correct — Dir.home is Ruby's official, well-tested cross-platform solution that handles edge cases properly. The old implementation had a hand-rolled fallback chain (HOME → HOMEDRIVE+HOMEPATH → APPDATA → USERPROFILE) that Ruby already handles correctly internally
3. Less maintenance burden — No need to maintain Windows-specific path normalization or environment variable logic that duplicates Ruby stdlib functionality
4. Consistent behavior — Aligns Rake's behavior with how other Ruby tools determine the home directory on Windows

Because it's hard to use and test something that isn't there anymore 😄 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.