Remove X11-specific wxWindowIsVisible() from common Unix file.

XGetWindowAttributes() used in this function resulted in link problems under
Fedora 14 because we don't explicitly link with libX11. Instead of doing this,
simply get rid of this function as it seems to be used in exactly one place in
wxX11 code only anyhow.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66885 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775