core_proxy_field_legacy: do not set GMT as default timezone !

bt5/erp5_core_proxy_field_legacy/SkinTemplateItem/portal_skins/erp5_core_proxy_field_legacy/Base_viewFieldLibrary/my_date.xml

@@ -271,7 +271,7 @@
                 </item>
                 <item>
                     <key> <string>default_timezone</string> </key>
-                    <value> <string>GMT</string> </value>
+                    <value> <string></string> </value>
                 </item>
                 <item>
                     <key> <string>description</string> </key>