ltp-testsuite: drop redundant arch dependency entries

BR2_TOOLCHAIN_HAS_SYNC_4 is enabled for all these architectures except
arc. There is no need to list them explicitly.

Cc: Romain Naour <romain.naour@gmail.com>
Cc: Petr Vorel <petr.vorel@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Acked-by: Petr Vorel <petr.vorel@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>