Revert "metadata: complain if link url isn't a url"

I already tried this once. It had to be reverted because Maintainer Notes
often don't obey this. I had forgotten.

This reverts commit 97ba94903e.

fdroidserver/metadata.py

@@ -335,8 +335,6 @@ class DescriptionFormatter:
                 if index == -1:
                     raise MetaDataException("Unterminated ]")
                 url = txt[1:index]
-                if ':' not in url:
-                    raise MetaDataException("Wrong link")
                 index2 = url.find(' ')
                 if index2 == -1:
                     urltxt = url