[Ada] Remove dead code for scope entity having E_Subprogram_Body kind

[Ada] Remove dead code for scope entity having E_Subprogram_Body kind

In GNAT AST the Scope field always points to the semantic scope (e.g.
subprogram) and never to syntactic scope (e.g. subprogram body).

Cleanup related to handling of circular access-to-record types.

gcc/ada/

	* gcc-interface/decl.cc (gnat_to_gnu_entity): Remove dead code
	which expected Scope to return E_Subprogram_Body entity.