input: Write a function to clean a string of HTML tags. The function should remove all HTML tags from the given 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("<html><body><h1>Title</h1><p>Text</p></body></html>")=="TitleText"
  assert clean_html_tags("<a href='link.html'>Click here</a>")=="Click here"
  assert clean_html_tags("<div>Some content</div>")=="Some content"
  assert clean_html_tags("No tags here")=="No tags here"
output: def clean_html_tags(s):
      import re
      return re.sub(r'<[^>]+>', '', s)