title: Drop coordinate from an xarray DataArray
body: <p>I have an xarray DataArray that looks like this below with shape (1,5,73,144,17) and I'm trying to drop or delete the &quot;level&quot; coordinates. So, ultimately, i need the variable to have shape = (1,5,73,144).</p>
  <pre><code>stdna
  Out[717]: 
  &lt;xarray.DataArray 'stack-6e9b86fc65e3f0fda2008a339e235bc7' (variable: 1, week: 5, lat: 73, lon: 144, 
  level: 17)&gt;
  dask.array&lt;stack, shape=(1, 5, 73, 144, 17), dtype=float32, chunksize=(1, 1, 73, 144, 17), 
  chunktype=numpy.ndarray&gt;
  Coordinates:
  * lon       (lon) float32 0.0 2.5 5.0 7.5 10.0 ... 350.0 352.5 355.0 357.5
  * lat       (lat) float32 90.0 87.5 85.0 82.5 80.0 ... -82.5 -85.0 -87.5 -90.0
  * level     (level) float32 1000.0 925.0 850.0 700.0 ... 50.0 30.0 20.0 10.0
  * week      (week) int64 5 6 7 8 9
  * variable  (variable) &lt;U3 'hgt'
  </code></pre>
  <p>I've taken a look to the xarray documentation and it's not helping. I've tried different combinations around this idea but i usually get the statement below and the coordinate has not been removed:</p>
  <pre><code>s = stdna.drop('level', dim=None)
  
  Dimensions without coordinates: level
  </code></pre>
  <p>Thank you for your help!</p>