Remove unused variables

This fixes PVS Studio static analyzer warnings:

V808 'errMsg' object of 'wxString' type was created but was not utilized. docview.cpp 1182
V808 'search' object of 'wxString' type was created but was not utilized. dirctrlg.cpp 697
V808 'filename' object of 'wxString' type was created but was not utilized. dirctrlg.cpp 697

(see full report at http://www.fly-server.ru/pvs-studio/wxWidgets-core/)

Closes https://github.com/wxWidgets/wxWidgets/pull/1096

src/common/docview.cpp

@@ -1179,7 +1179,6 @@ void wxDocManager::DoOpenMRUFile(unsigned n)
     if ( filename.empty() )
         return;
 
-    wxString errMsg; // must contain exactly one "%s" if non-empty
     if ( wxFile::Exists(filename) )
     {
         // Try to open it but don't give an error if it failed: this could be

src/generic/dirctrlg.cpp

@@ -694,7 +694,7 @@ void wxGenericDirCtrl::PopulateNode(wxTreeItemId parentId)
 
     wxASSERT(data);
 
-    wxString search,path,filename;
+    wxString path;
 
     wxString dirName(data->m_path);