manual qa: drop ltpstress test

manual qa: drop ltpstress test

ltpstress was removed in Oct 2018

https://github.com/linux-test-project/ltp/commit/e752f7c19674d9b2f7d37aed123561a3f6410e97#diff-5231627fc8640e0adb955f9e69c3c08d

Remove LTP stress tests
ltpstress.sh runs stress.part[1-3]. But these runtest files just
duplicate definitions:
* stress.part1: fs, mm, nfs
* stress.part2: ipc, math, nptl
* stress.part3: net.multicast, pty, syscalls

The definitions are outdated anyway. There is no point trying
to keep them sync.

Signed-off-by: Armin Kuster <akuster808@gmail.com>