record_section_with_aarch64_elf_section_dat

Nowhere in the aarch64 backend is the list created by this function
examined, and in any case there are much simpler ways to determine the
type of elf_section_data attached to a bfd ELF section.  It will
always be according to the target_id of the section owner.

Delete sections_with_aarch64_elf_section_data and everything
associated with it.