tests: remove test case du/slink

tests: remove test case du/slink

This test tried to ensure that not all symlinks (across all
file system types) have Zero size and refers to a change
in system.h from 2002-08-31 (commit SH-UTILS-2_0_15-55-g62808a7).
The test used to do this by working on symlinks to long file
names.  This assumption is dependant on the underlying file
system, and in some environments like XEN does not even work
on file systems known to work otherwise.

The test for dereferencing and no-dereferencing symlinks is
already covered by other tests (du/deref.sh, du/deref-args.sh,
and du/no-deref.sh).  Therefore, remove it.

* tests/du/slink.sh: Remove file.
* tests/local.mk (all_tests): Remove the above test.

Discussed in:
http://lists.gnu.org/archive/html/coreutils/2013-01/msg00053.html