misc: remove tcore testcase

After a little bit of internet archeology and it seems that patch which
introduced /proc/sys/kernel/core_dump_threads file never got accepted
into mainline linux kernel. (I found some discussion about it around
linux 2.4 and 2.5 development.)

So it looks like this test was never working on mainline kernels.

Reported-by: C Nandi <ccnandi@yahoo.com>
Signed-off-by: Cyril Hrubis <chrubis@suse.cz>