Remove has_separator property from gtkbuilder files

pluma/dialogs/pluma-encodings-dialog.ui

@@ -10,7 +10,6 @@
     <property name="modal">True</property>
     <property name="resizable">True</property>
     <property name="destroy_with_parent">True</property>
-    <property name="has_separator">False</property>
     <child internal-child="vbox">
       <object class="GtkVBox" id="dialog-vbox3">
         <property name="visible">True</property>

pluma/dialogs/pluma-preferences-dialog.ui

@@ -31,7 +31,6 @@
     <property name="resizable">False</property>
     <property name="destroy_with_parent">True</property>
     <property name="type_hint">dialog</property>
-    <property name="has_separator">False</property>
     <child internal-child="vbox">
       <object class="GtkVBox" id="dialog-vbox">
         <property name="visible">True</property>

pluma/dialogs/pluma-search-dialog.ui

@@ -15,7 +15,6 @@
     <property name="gravity">GDK_GRAVITY_NORTH_WEST</property>
     <property name="focus_on_map">True</property>
     <property name="urgency_hint">False</property>
-    <property name="has_separator">False</property>
     <child internal-child="vbox">
       <object class="GtkVBox" id="dialog-vbox1">
         <property name="visible">True</property>