Fix bogus asserts in Unix wxFileSystemWatcher for removed files.

Fix bogus asserts in Unix wxFileSystemWatcher for removed files.

Store the recently removed file descriptors and don't assert if we get an
event for one of them, this can happen and is normal unlike unexpected events
for completely unknown descriptors for which we still keep an assert.

Closes #14465.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72049 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775