Remove .cvsignore files from expat sources.

These files shouldn't be included in the distribution and it's simpler to just
remove them, as it is pretty unlikely that a new Expat release is going to be
made, and even less likely that it would still use CVS if it does happen, than
to filter them out from the distribution creation scripts.

Closes #16711.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78253 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775