Remove rendering of literate Haskell.

This was breaking display of a lot of code files: see #196.

The possibility of auto-detecting lhs/Markdown files and only rendering them was suggested in the discussion on #196. However, the literate Haskell format lacks a "this is the markup flavour we're using" directive, and trying to guess the correct rendering engine based on heuristics strikes me as error-prone. Removing lhs rendering entirely is the simplest solution, and it also allows the use of GitHub's code-browsing tools like line number links on lhs files.