Revert "arm64: kexec: make dtb_mem always enabled"

Adding crash dump support to 'kexec_file' is going to extend 'struct
kimage_arch' with more 'kexec_file'-specific members. The cleanup here
then starts to get in the way, so revert it.

This reverts commit 62151678.

Reported-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Will Deacon <will@kernel.org>