Revert "[System.IO] Phase 2 of revamping KeventWatcher (FileSystemWatcher implementation for OS X/kqueue). Dozens of bugs and reliability issues fixed."

This reverts commit 717c6de1cc49dabb2ecaa2a844a8b2b162c21440.

The new FSW is not making the 3.12 release.
Revert "[System.IO] Reimplemented much of the kqueue-based file watcher so that watching subdirectories works."

This reverts commit 13fd028ee87066b157b66904f8cc7ee8d8cf7181.

The new FSW is not making the 3.12 release.