asm-generic: Remove empty #ifdef SA_RESTORER

There was a #ifdef SA_RESTORER to guard the sa_restorer field
in struct sigaction.

Commit 8a1ab315 ("UAPI: (Scripted) Disintegrate
include/asm-generic") moved that struct into
uapi/asm-generic/signal.h but the #ifdef SA_RESTORER remained.

Remove it.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>