LoongArch: Remove unused/useless definitions.

LoongArch: Remove unused/useless definitions.

This patch removes some unnecessary definitions of target hook functions
according to the documentation of GCC.

gcc/ChangeLog:

	* config/loongarch/loongarch-protos.h
	(loongarch_cfun_has_cprestore_slot_p): Delete.
	(loongarch_adjust_insn_length): Delete.
	(current_section_name): Delete.
	(loongarch_split_symbol_type): Delete.
	* config/loongarch/loongarch.cc
	(loongarch_case_values_threshold): Delete.
	(loongarch_spill_class): Delete.
	(TARGET_OPTAB_SUPPORTED_P): Delete.
	(TARGET_CASE_VALUES_THRESHOLD): Delete.
	(TARGET_SPILL_CLASS): Delete.