Remove two unused variables:

  arch/x86/kernel/process.c: In function ‘__switch_to_xtra’:
  arch/x86/kernel/process.c:618:31: warning: variable ‘next’ set but not used [-Wunused-but-set-variable]
    618 |  struct thread_struct *prev, *next;
        |                               ^~~~
  arch/x86/kernel/process.c:618:24: warning: variable ‘prev’ set but not used [-Wunused-but-set-variable]
    618 |  struct thread_struct *prev, *next;
        |

They are never used and so can be removed.
Signed-off-by: yu kuai <yukuai3@huawei.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: x86-ml <x86@kernel.org>
Cc: yi.zhang@huawei.com
Cc: zhengbin13@huawei.com
Link: https://lkml.kernel.org/r/20191213121253.10072-1-yukuai3@huawei.com