title: how to drop a pandas multi level dataframe column when all sub columns are completely blank
body: <p><strong>dataframe:</strong></p>
  <pre><code>|--------------------------------------------------------------------|
  |    Name        |    email          |  Phone no    |   Gender       |
  |----------------|-------------------|--------------|----------------|
  |legacy | target |legacy    | target |legacy|target |legacy | target |
  |-------|--------|----------|--------|------|-------|-------|--------|
  |Name1  |Name1   |n1@abc.com|        |      |       |       |        |
  |Name2  |Name2   |          |        |      |   12  |       |        |
  |--------------------------------------------------------------------|
  </code></pre>
  <p><strong>Expected output:</strong></p>
  <pre><code>|---------------------------------------------------|
  |    Name        |    email          |  Phone no    |
  |----------------|-------------------|--------------|
  |legacy | target |legacy    | target |legacy|target |
  |-------|--------|----------|--------|------|-------|
  |Name1  |Name1   |n1@abc.com|        |      |       |
  |Name2  |Name2   |          |        |      |   12  |
  |---------------------------------------------------|
  </code></pre>
  <p>I am using the below code, but it is removing &quot;email target&quot; and &quot;phone no legacy&quot; column as well.</p>
  <pre><code>df.dropna(how='all', axis=1, inplace=True)
  </code></pre>
  <p>However I want to drop only the &quot;Gender&quot; column as this is the only column where both legacy and target fields are completely blank.</p>
  <p>Could anyone please help me.</p>
  <p>Thank you.</p>