efi: vars: Drop __efivar_entry_iter() helper which is no longer used

__efivar_entry_iter() uses a list iterator in a dubious way, i.e., it
assumes that the iteration variable always points to an object of the
appropriate type, even if the list traversal exhausts the list
completely, in which case it will point somewhere in the vicinity of the
list's anchor instead.

Fortunately, we no longer use this function so we can just get rid of it
entirely.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>