title: Remove extra legends in ggplot2 when there are multiple
body: <p>For example:</p>
  
  <pre><code>require(ggplot2)
  require(reshape2)
  volcano3d &lt;- melt(volcano) 
  names(volcano3d) &lt;- c("x", "y", "z") 
  v &lt;- ggplot(volcano3d, aes(x, y, z = z)) 
  v1 = v +  stat_contour(aes(colour=..level..,size=..level..)) 
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/hJLL8.png" alt="enter image description here"></p>
  
  <p>There are two legends on the side, can I remove one of them?</p>