ltp-testsuite: don't select libtirpc

Since commit 8f1c4c49 (ltp-testsuite: don't link with libtirpc)
ltp-testsuite does not need libtirpc. Don't select it.

Cc: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>