arm64: Remove __exception_text_start and __exception_text_end from asm/section.h

Linux commit b6e43c0e

 ("arm64: remove __exception annotations") has
removed __exception_text_start and __exception_text_end sections.

So removing reference of __exception_text_start and __exception_text_end
from from asm/section.h.

Cc: James Morse <james.morse@arm.com>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Prabhakar Kushwaha <pkushwaha@marvell.com>
Signed-off-by: Will Deacon <will@kernel.org>