arm64: Removed unused variable

arch/arm64/kernel/signal.c: In function ‘handle_signal’:
arch/arm64/kernel/signal.c:290:22: warning: unused variable ‘thread’ [-Wunused-variable]

Fixes: arm64: Remove signal translation and exec_domain
Reported-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>