Deprecate scrapy.python.utils.FixedSGMLParser

SGML link extractor that uses it is also going to be deprecated;
sgmllib module is not available in Python 3.
deprecate SgmlLinkExtractor
FixedSGMLParser is useless.
Remove FixedSGMLParser: it is both useless and unused.
Merge pull request #777 from scrapy/deprecate-sgml

Deprecate SgmlLinkExtractor