input: Write a function to clean a string of HTML tags. The function should remove all HTML tags from the input string and return the cleaned string.
  assert clean_html_tags("<p>This is a <b>test</b> string</p>")=='This is a test string'
  assert clean_html_tags("<div><h1>Header</h1><p>Paragraph</p></div>")=='HeaderParagraph'
  assert clean_html_tags("No tags here")=='No tags here'
  assert clean_html_tags("")==''
output: def clean_html_tags(s):
      import re
      return re.sub(r'<[^>]+>', '', s)