core: arm: Remove duplicated sp assignment for ARM64 in set_ctx_regs()

There are two lines of code to assign value of sp for ARM64. Remove one
of them.

Signed-off-by: Alvin Chang <alvinga@andestech.com>
Reviewed-by: Etienne Carriere <etienne.carriere@foss.st.com>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>

core/arch/arm/kernel/thread.c

@@ -981,7 +981,6 @@ static void set_ctx_regs(struct thread_ctx_regs *regs, unsigned long a0,
 	regs->x[1] = a1;
 	regs->x[2] = a2;
 	regs->x[3] = a3;
-	regs->sp = user_sp;
 	regs->pc = entry_func;
 	regs->cpsr = spsr;
 	regs->x[13] = user_sp;	/* Used when running TA in Aarch32 */