title: Openpyxl not removing sheets from created workbook correctly
body: <p>So I ran into an issue with <code>remove_sheet()</code> with openpxyl that I can't find an answer to. When I run the following code:</p>
  
  <pre><code>import openpyxl
  
  wb = openpyxl.Workbook()
  ws = wb.create_sheet("Sheet2")
  wb.get_sheet_names()
  ['Sheet','Sheet2']
  wb.remove_sheet('Sheet')     
  </code></pre>
  
  <p>I get the following error:</p>
  
  <pre><code>ValueError: list.remove(x): x not in list
  </code></pre>
  
  <p>It doesn't work, even if I try <code>wb.remove_sheet(0)</code> or <code>wb.remove_sheet(1)</code>, I get the same error. Is there something I am missing?</p>