title: randomly remove rows from dataframe based on condition
body: <p>given a dataframe with numerical values in a specific column, I want to randomly remove a certain percentage of the rows for which the value in that specific column lies within a certain range.</p>
  
  <p>For example given the following dataframe:</p>
  
  <pre><code>df = pd.DataFrame({'col1': [1,2,3,4,5,6,7,8,9,10]})
  df
     col1
  0     1
  1     2
  2     3
  3     4
  4     5
  5     6
  6     7
  7     8
  8     9
  9    10
  </code></pre>
  
  <p>2/5 of the rows where col1 is below 6 should be removed randomly.</p>
  
  <p>Whats the most concise way to do that?</p>