Remove current implementation of WatchFlags

removes fsnFlags, purgeEvents and internalEvent channel

* current implementation doesn't take advantage of OS for efficiency
* provides little benefit over filtering events as they are received, but has  extra bookkeeping and mutexes
* no tests for the current implementation
* not fully implemented on Windows https://github.com/howeyc/fsnotify/issues/93#issuecomment-39285195

TODO: go.exp CL for this.