Bug 1567424 - remove some leftover broadcaster references. r=jorgk

mailnews/base/prefs/content/am-identity-edit.xul

@@ -127,11 +127,10 @@
             <textbox id="identity.signature" datatype="nsIFile"
                      flex="1" name="identity.signature"
                      aria-labelledby="identity.attachSignature"
-                     observes="broadcaster_attachSignature"
                      class="uri-element"/>
             <button id="identity.sigbrowsebutton" class="push"
                     label="&choose.label;" accesskey="&choose.accesskey;"
-                    oncommand="selectFile()" observes="broadcaster_attachSignature"/>
+                    oncommand="selectFile()"/>
           </hbox>
 
           <hbox align="center">