arm64: traps: remove duplicate include statement

asm/exception.h is included more than once. Remove the one that isn't
necessary.

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Link: https://lore.kernel.org/r/1609139108-10819-1-git-send-email-tiantao6@hisilicon.com


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>