Drop installcheck target

The installcheck target is supposed to be used to run tests on the
installed files, however our current implementation runs the exact same
tests in the build directory as the check target, which makes it useless.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>