Remove is_n64_ data member from Mips_relobj class.

Remove is_n64_ data member from Mips_relobj class.

elfcpp/
	* mips.h (abi_64): Remove.

gold/
	* mips.cc (Mips_relobj::is_n64_): Remove.
	(Target_mips::ei_class_): Likewise.
	(Mips_relobj::is_newabi): Call methods.
	(Mips_relobj::is_n64): Change checking for N64 ABI.
	(Target_mips::is_output_n64): Likewise.
	(Target_mips::merge_processor_specific_flags): Remove ei_class
	argument, and remove comparing ei_class.
	(Target_mips::do_adjust_elf_header): Remove setting EI_CLASS field
	of the ELF header.
	(Target_mips::do_finalize_sections): Don't pass ei_class argument
	to merge_processor_specific_flags.
	(Target_mips::elf_mips_abi_name): Remove ei_class argument, and
	change checking for N64 ABI.