Delete hack when takeDirectory returns ""

Summary:
Since commits 8fe1f8 and bb6731 in the filepath packages (ticket #2034, closed
in 2010), takeDirectory "foo" returns ".", and not "", so this check is no
longer needed.

Other commits:
* Remove trailing whitespace
* Update comments for #2278

Test Plan: harbormaster

Reviewers: austin

Reviewed By: austin

Subscribers: simonmar, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D213

GHC Trac Issues: #2034