news: Drop empty sections

We have occasionally failed to document certain categories
of changes in the release notes, yet still left the
corresponding sections in the file even though they were
completely empty.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>

docs/news.xml

@@ -701,8 +701,6 @@
         </description>
       </change>
     </section>
-    <section title="Improvements">
-    </section>
     <section title="Bug fixes">
       <change>
         <summary>
@@ -1010,8 +1008,6 @@
         </description>
       </change>
     </section>
-    <section title="Bug fixes">
-    </section>
   </release>
   <release version="v4.5.0" date="2018-07-02">
     <section title="New features">
@@ -1325,8 +1321,6 @@
         </description>
       </change>
     </section>
-    <section title="Bug fixes">
-    </section>
   </release>
   <release version="v4.3.0" date="2018-05-02">
     <section title="New features">