title: Remove certain string from entire column in pandas dataframe
body: <p>I have a pandas dataframe <code>df</code> with the contents below:</p>
  
  <pre><code>  Date          Factor       Expiry         Grade  
  0 12/31/1991    2.138766     3/30/1992      -3.33% 
  1 10/29/1992    2.031381     2/8/1993       -1.06% 
  2 5/20/1993     2.075670     6/4/1993       -6.38% 
  </code></pre>
  
  <p>I would like the remove the <code>%</code> character from all the rows in the <code>Grade</code> column. The result should look like this:</p>
  
  <pre><code>  Date          Factor     Expiry        Grade  
  0 12/31/1991    2.138766   3/30/1992     -3.33 
  1 10/29/1992    2.031381   2/8/1993      -1.06 
  2 5/20/1993     2.075670   6/4/1993      -6.38 
  </code></pre>
  
  <p>I am using Python v3.6.</p>