Remove *.in files from MANIFEST.in

There no longer appear to be any *.in files in this project, presumably
because we switched to Jinja2 templates instead. Refs #1155

MANIFEST.in

@@ -1,3 +1,3 @@
 include *.rst
-recursive-include pelican *.html *.css *png *.in *.rst *.markdown *.md *.mkd *.xml *.py
+recursive-include pelican *.html *.css *png *.rst *.markdown *.md *.mkd *.xml *.py
 include LICENSE THANKS docs/changelog.rst