ada: Remove unused AST flag Address_Warning_Posted

Flag Address_Warning_Posted was only read and never written, so it can be
safely removed.

gcc/ada/ChangeLog:

	* gen_il-fields.ads (Opt_Field_Enum): Remove flag.
	* gen_il-gen-gen_nodes.adb (N_Attribute_Definition_Clause): Remove
	field.
	* sem_ch13.adb (Validate_Address_Clauses): Remove read of the flag.
	* sinfo.ads (Address_Warning_Posted): Remove flag description.