Don't make wxWebRequest::SplitParameters() public

They're not necessary to use this class and we may consider exporting
them later, possibly with a better API and more tests, if really needed.

Also do change their API slightly by leaving only a single function and
returning the value instead of using an out parameter for it to make it
simpler to use.

include/wx/webrequest.h

@@ -93,12 +93,6 @@ public:
 
     void ReportDataReceived(size_t sizeReceived);
 
-    static void SplitParameters(const wxString& s, wxString& value,
-        wxWebRequestHeaderMap& parameters);
-
-    static void SplitParameters(const wxString::const_iterator& begin,
-        const wxString::const_iterator& end, wxWebRequestHeaderMap& parameters);
-
 protected:
     wxString m_method;
     Storage m_storage;