Remove default argument value from wxIHTMLElement2::setExpression().

This doesn't make much sense for an interface declaration anyhow and produces
a warning from g++ about assigning string literal to (non-const) BSTR pointer.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77076 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775