arm64_syscall.c: Don't need to set register context during syscall

The register context is not needed, the original idea was to provide
the user stack pointer for signal handler delivery, but the user stack
can be obtained via sp_el0 so the context registers are not needed.

SP0 is not stored upon exception entry anyways, so this code is just
completely redundant and wrong.

arch/arm64/src/common/arm64_syscall.c

@@ -94,7 +94,7 @@ static void  arm64_dump_syscall(const char *tag, uint64_t cmd,
 uintptr_t dispatch_syscall(unsigned int nbr, uintptr_t parm1,
                            uintptr_t parm2, uintptr_t parm3,
                            uintptr_t parm4, uintptr_t parm5,
-                           uintptr_t parm6, void *context)
+                           uintptr_t parm6)
 {
   struct tcb_s *rtcb         = this_task();
   register long x0 asm("x0") = (long)(nbr);
@@ -116,10 +116,6 @@ uintptr_t dispatch_syscall(unsigned int nbr, uintptr_t parm1,
       return -ENOSYS;
     }
 
-  /* Set the user register context to TCB */
-
-  rtcb->xcp.regs = context;
-
   /* Indicate that we are in a syscall handler */
 
   rtcb->flags |= TCB_FLAG_SYSCALL;

arch/arm64/src/common/arm64_vectors.S

@@ -239,7 +239,6 @@ SECTION_FUNC(text, arm64_sync_exc)
     /* Call dispatch_syscall() on the kernel stack with interrupts enabled */
 
     enable_irq
-    mov    x7, sp /* x7 = context */
     bl     dispatch_syscall
     disable_irq