arm64: kdump: Remove custom linux,elfcorehdr handling

Remove the architecture-specific code for handling the
"linux,elfcorehdr" property under the "/chosen" node in DT, as the
platform-agnostic handling in the FDT core code already takes care of
this.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/3b8f801f9b92066855e87f3079fafc153ab20f69.1628670468.git.geert+renesas@glider.be