ada: Remove flag Is_Inherited_Pragma which is only set and never used

Code cleanup; behavior is unaffected. Flag Is_Inherited_Pragma is only set in
GNAT, but is not actually used, neither by the compiler nor by any backend.

gcc/ada/ChangeLog:

	* contracts.adb (Inherit_Pragma): Don't set flag Is_Inherited_Pragma.
	* gen_il-fields.ads (Opt_Field_Enum): Remove field identifier.
	* gen_il-gen-gen_nodes.adb (N_Pragma): Remove field from node.
	* sinfo.ads (Is_Inherited_Pragma): Remove field description.
	(N_Pragma): Remove field reference.