arm64: Remove documentation about TIF_USEDFPU

TIF_USEDFPU is not defined as thread flags for Arm64. So drop it from
the documentation.
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Julien Grall <julien.grall@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/thread_info.h

@@ -79,7 +79,6 @@ void arch_release_task_struct(struct task_struct *tsk);
  *  TIF_SIGPENDING	- signal pending
  *  TIF_NEED_RESCHED	- rescheduling necessary
  *  TIF_NOTIFY_RESUME	- callback before returning to user
- *  TIF_USEDFPU		- FPU was used by this task this quantum (SMP)
  */
 #define TIF_SIGPENDING		0
 #define TIF_NEED_RESCHED	1