efi_loader: remove weak efi_get_public_key_data()

Configuring a system with CONFIG_EFI_CAPSULE_AUTHENTICATE=y but without a
method to retrieve the public key data is a misconfiguration. We should get
a build failure for it. Therefore remove the weak efi_get_public_key_data()
implementation.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>