input: Write a function to remove all HTML tags from a given string. If there are no HTML tags, return the string as is.
  assert clean_html_tags("<p>This is a <b>test</b> string</p>")=="This is a test string"
  assert clean_html_tags("")==""
  assert clean_html_tags("This is a simple string without tags")=="This is a simple string without tags"
  assert clean_html_tags("<a href='example.com'>Link</a>")=="Link"
  assert clean_html_tags(" <div> <p> Hello </p> </div> ")=="  Hello  "
output: def clean_html_tags(s):
      import re
      return re.sub(r'<[^>]+>', '', s)