Remove download_url from setup.py (#1164)

This is an old, outdated key which is simply moved into `project_urls`. As it offers no value, it is being removed.
Fixes #1163.

Author: waylan

setup.py

@@ -59,7 +59,6 @@ def get_version():
     name='Markdown',
     version=__version__,
     url='https://Python-Markdown.github.io/',
-    download_url='http://pypi.python.org/packages/source/M/Markdown/Markdown-%s-py2.py3-none-any.whl' % __version__,
     project_urls={
         'Documentation': 'https://Python-Markdown.github.io/',
         'GitHub Project': 'https://github.com/Python-Markdown/markdown',