Delete support for deprecated "-- # ..."-style haddock options

A long time ago, you could use `"-- # <haddock options>"` to mean that
`<haddock options.` should be passed to `haddock`. Since 2007
(03d8585e0940e28e024548654fe3505685aca94f), using `OPTIONS_HADDOCK` is
the preferred way to do this.

Why is ok to remove support for "-- # .."?

* It is not mentioned in the Haddock user's guide, nor are there any
  tests that use it.

* Ever since 2011 (b3e30449aa6d6eaa978eb3c7447ca85985d9d251), it doesn't
  actually work anymore. The function `getOptionsFromFile` uses
  `gopt_unset dflags Opt_Haddock` for other reasons, so even when
  running ghc with `--haddock`, the following rule always fires when the
  lexer sees "-- # ..", and it gets treated as a normal comment:

```
    -- Next, match Haddock comments if no -haddock flag
    "-- " [$docsym \#] .* / { ifExtension (not . haddockEnabled) } { lineCommentToken }
```

Reviewed by: bgamari

Differential Revision: https://phabricator.haskell.org/D1932

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