Deprecate SgmlLinkExtractor

As discussed in https://github.com/scrapy/scrapy/pull/756
FixedSGMLParser was a part of public interface (class and module names without `_`, a docstring), so I added a deprecation warning instead of removing it. SgmlLinkExtactor shows a single warning in this PR (because it is no longer using FixedSGMLParser). 

But neither github search nor our internal search shows any FixedSGMLParser usages indeed. Let's remove it!