User profile backups: Don't backup user galleries

As suggested in
https://lists.freedesktop.org/archives/libreoffice/2017-January/076768.html

Change-Id: I2562064ae906eb8b2d507ec971762625692a4bbf
Reviewed-on: https://gerrit.libreoffice.org/33608Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>
Tested-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>

comphelper/source/misc/backupfilehelper.cxx

@@ -2693,9 +2693,6 @@ namespace comphelper
             // Initial and User-defined Databases
             maDirs.insert("database");
 
-            // User-Defined Galleries
-            maDirs.insert("gallery");
-
             // most part of registry files
             maDirs.insert("registry");