Simplify and fix "removeComments()"

Created by: Ede123

• The separate treatment of comments at the documentElement's level is not necessary - they have a parent (as tested in Python 3.5.0 and 2.7.11 and 2.6.6)! It might not have worked before due to a typo - note the "if isinstance(element,...)" and "len(element.data)" which should both refer to "subelement" instead - or a bug in very old versions of Python).
• Fix the iteration over childNodes (i.e. replace "for subelement in element.childNodes:" with ""for subelement in element.childNodes[:]:". We have to create a copy of the list to iterate over, otherwise we'd be iterating over a list as we change it which leads to unpredictable results.

Fixes #25 (closed)