Drop imagesy() return false description

Closes GH-2202.

Author: othercorey

reference/image/functions/imagesy.xml

@@ -26,8 +26,7 @@
  <refsect1 role="returnvalues">
   &reftitle.returnvalues;
   <para>
-   Return the height of the <parameter>image</parameter> or &false; on 
-   errors.
+   Return the height of the <parameter>image</parameter>.
   </para>
  </refsect1>