ada: Get rid of unused node/entity fields (neither set nor get)

ada: Get rid of unused node/entity fields (neither set nor get)

The following fields are not used in GNAT nor in any of its clients.
Neither the setters nor the getters are called.

    Node fields:
        Do_Storage_Check
        Enclosing_Variant
        Handler_List_Entry
        Is_Subprogram_Descriptor
        Renaming_Exception
        Default_Expression

    Entity fields:
        Is_Unimplemented

This patch removes all of them.

gcc/ada/ChangeLog:

	* einfo.ads: Remove unused fields.
	* gen_il-fields.ads: Likewise.
	* gen_il-gen-gen_entities.adb: Likewise.
	* gen_il-gen-gen_nodes.adb: Likewise.
	* sinfo.ads: Likewise.
	* gen_il-gen.adb (Compile): Remove obsolete comment.