Simplify elf64_alpha_relocate_section

Prior to Nick implementing _bfd_elf_make_dynamic_reloc_section, alpha
made no use of elf_section_data(sec)->sreloc to store the dynamic
relocation section associated with sec.  After commit 83bac4b01082 the
elf_section_data was set but not used in relocate_section.

	* elf64-alpha.c (elf64_alpha_relocate_section): Use
	elf_section_data to get dynamic relocation section associated
	with the input section.