Fix wxFileSystemWatcher::Remove() in wxMSW.

Fix wxFileSystemWatcher::Remove() in wxMSW.

Removing the path watched by wxFileSystemWatcher didn't do anything in wxMSW
implementation so we still continued getting events for the changes to this
path even after calling Remove().

Fix this by really implementing Remove() properly. Also add a unit test
checking that we don't get any events after calling Remove().

See #12847.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67691 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775