Codechange: Don't store tree counter in the map array

(cherry picked from commit 6c3f351d5a4b0d19a1a265f04f44c70c24c19cd2)

See: https://github.com/OpenTTD/OpenTTD/pull/10018

docs/landscape.html

@@ -840,8 +840,6 @@
       </table>
      </li>
      <li>m2 bits 5..4: ground density</li>
-     <li>m2 bits 3..0: update counter, incremented on every periodic processing.<br>
-          on wraparound the growth status is updated (or, if it's <tt>3</tt>, a random action is taken)</li>
      <li>m3 bits 7..0: type of trees:
       <table>
        <tr>