OpenMP: Remove dead code from declare variant reimplementation

After reimplementing late resolution of "declare variant", the
declare_variant_alt and calls_declare_variant_alt flags on struct
cgraph_node are no longer used by anything.  For the purposes of
marking functions that need late resolution, the
has_omp_variant_constructs flag has replaced
calls_declare_variant_alt.

Likewise struct omp_declare_variant_entry, struct
omp_declare_variant_base_entry, and the hash tables used to store
these structures are no longer needed, since the information needed for
late resolution is now stored in the gomp_variant_construct nodes.

In addition, some obsolete code that was temporarily ifdef'ed out
instead of delted in order to produce a more readable patch for the
previous installment of this series is now removed entirely.

There are no functional changes in this patch, just removing dead code.

gcc/ChangeLog
	* cgraph.cc (symbol_table::create_edge): Don't set
	calls_declare_variant_alt in the caller.
	* cgraph.h (struct cgraph_node): Remove declare_variant_alt
	and calls_declare_variant_alt flags.
	* cgraphclones.cc (cgraph_node::create_clone): Don't copy
	calls_declare_variant_alt bit.
	* gimplify.cc: Remove previously #ifdef-ed out code.
	* ipa-free-lang-data.cc (free_lang_data_in_decl): Adjust code
	referencing declare_variant_alt bit.
	* ipa.cc (symbol_table::remove_unreachable_nodes): Likewise.
	* lto-cgraph.cc (lto_output_node): Remove references to deleted
	bits.
	(output_refs): Adjust code referencing declare_variant_alt bit.
	(input_overwrite_node): Remove references to deleted bits.
	(input_refs): Adjust code referencing declare_variant_alt bit.
	* lto-streamer-out.cc (lto_output): Likewise.
	* lto-streamer.h (omp_lto_output_declare_variant_alt): Delete.
	(omp_lto_input_declare_variant_alt): Delete.
	* omp-expand.cc (expand_omp_target): Use has_omp_variant_constructs
	bit to trigger pass_omp_device_lower instead of
	calls_declare_variant_alt.
	* omp-general.cc (struct omp_declare_variant_entry): Delete.
	(struct omp_declare_variant_base_entry): Delete.
	(struct omp_declare_variant_hasher): Delete.
	(omp_declare_variant_hasher::hash): Delete.
	(omp_declare_variant_hasher::equal): Delete.
	(omp_declare_variants): Delete.
	(omp_declare_variant_alt_hasher): Delete.
	(omp_declare_variant_alt_hasher::hash): Delete.
	(omp_declare_variant_alt_hasher::equal): Delete.
	(omp_declare_variant_alt): Delete.
	(omp_lto_output_declare_variant_alt): Delete.
	(omp_lto_input_declare_variant_alt): Delete.
	(includes): Delete unnecessary include of gt-omp-general.h.
	* omp-offload.cc (execute_omp_device_lower): Remove references
	to deleted bit.
	(pass_omp_device_lower::gate): Likewise.
	* omp-simd-clone.cc (simd_clone_create): Likewise.
	* passes.cc (ipa_write_summaries): Likeise.
	* symtab.cc (symtab_node::get_partitioning_class): Likewise.
	* tree-inline.cc (expand_call_inline): Likewise.
	(tree_function_versioning): Likewise.

gcc/lto/ChangeLog
	* lto-partition.cc (lto_balanced_map): Adjust code referencing
	deleted declare_variant_alt bit.