real/Makefile: remove OTHER_SUBDIRS

Summary:
This removes the unnecessary `OTHER_SUBDIRS` variable, since the
subdirectories mentioned there are already in `SUBDIRS` (the
benchmarks have been enabled in previous commits).

Signed-off-by: Michal Terepeta <michal.terepeta@gmail.com>

Test Plan: build & run

Reviewers: bgamari, dfeuer

Reviewed By: dfeuer

Differential Revision: https://phabricator.haskell.org/D3342