Remove empty direction checkbox tooltips

The individual Direction checkboxes had empty tooltips. Removing them
means that the main section tooltip appears when you hover a mouse over
either of the individual checkboxes, but this seems like it could actually be
useful.

scripts/Interface/ui/sans_isis/beam_centre.ui

@@ -220,7 +220,7 @@
             <item>
              <widget class="QCheckBox" name="left_right_check_box">
               <property name="toolTip">
-               <string>&lt;html&gt;&lt;head/&gt;&lt;body&gt;&lt;p&gt;&lt;br/&gt;&lt;/p&gt;&lt;/body&gt;&lt;/html&gt;</string>
+               <string/>
               </property>
               <property name="text">
                <string>Left/Right</string>
@@ -230,7 +230,7 @@
             <item>
              <widget class="QCheckBox" name="up_down_check_box">
               <property name="toolTip">
-               <string>&lt;html&gt;&lt;head/&gt;&lt;body&gt;&lt;p&gt;&lt;br/&gt;&lt;/p&gt;&lt;/body&gt;&lt;/html&gt;</string>
+               <string/>
               </property>
               <property name="text">
                <string>Up/Down</string>