arm64/kernel: vmlinux.lds: drop redundant discard/keep macros

ARM_EXIT_KEEP and ARM_EXIT_DISCARD are always defined in the same way,
so we don't really need them in the first place.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20200416132730.25290-1-ardb@kernel.org


Signed-off-by: Will Deacon <will@kernel.org>