Remove one copy of duplicated alpha4 release note.

Noted by Andy Colson

doc/src/sgml/release-9.1.sgml

@@ -612,18 +612,6 @@
         </para>
        </listitem>
 
-       <listitem>
-        <para>
-         <emphasis>When an autovacuum worker (other than one performing an
-         anti-wraparound vacuum) is unable to obtain a lock on the target
-         relation without blocking, skip the relation.</emphasis>
-        </para>
-        <para>
-         This avoids pinning down an autovacuum worker.  The next autovacuum
-         cycle will try again. 
-        </para>
-       </listitem>
-
        <listitem>
         <para>
          <emphasis>Track time of last statistics reset on databases and