KDE5FilePicker: Drop (now) unused member '_helperMutex'

Change-Id: I4680cc4a8507d0d2409301b62ab81165fcfcdba1
Reviewed-on: https://gerrit.libreoffice.org/68047
Tested-by: Jenkins
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>
(cherry picked from commit 6d00c53f)
Reviewed-on: https://gerrit.libreoffice.org/72644

vcl/unx/kde5/KDE5FilePicker.hxx

@@ -31,7 +31,6 @@
 #include <com/sun/star/uno/XComponentContext.hpp>
 
 #include <osl/conditn.hxx>
-#include <osl/mutex.hxx>
 #include <rtl/ustrbuf.hxx>
 
 #include <functional>
@@ -51,8 +50,6 @@ class KDE5FilePicker : public Qt5FilePicker
 {
     Q_OBJECT
 protected:
-    osl::Mutex _helperMutex;
-
     //running filter string to add to dialog
     QStringList _filters;
     // map of filter titles to full filter for selection