Revert "doc: Vendorize alabaster Sphinx theme"

This ended up causes far too many issues to be worthwhile. We'll just
have to live with inconsistent haddock documentation.

This reverts commit cec21957.

.gitmodules

-[submodule "doc/alabaster"]
-	path = doc/alabaster
-	url = git://github.com/bitprophet/alabaster.git

alabaster @ aabda118

-Subproject commit aabda1182083d21b6d9bafb53ea8a99eb4be4cf0

doc/conf.py

@@ -29,7 +29,6 @@ pygments_style = 'tango'
 
 # -- Options for HTML output ----------------------------------------------
 
-html_theme_path = ['./alabaster']
 html_theme = 'alabaster'
 htmlhelp_basename = 'Haddockdoc'