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>