efi/esrt: remove esre_attribute::store()

esre_attribute::store() is not needed since commit af97a77b (efi:
Move some sysfs files to be read-only by root). Drop it.

Found by https://github.com/jirislaby/clang-struct

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Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: linux-efi@vger.kernel.org
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>