[Ada] Deconstruct unused Withed_Body filed of N_With_Clause node

[Ada] Deconstruct unused Withed_Body filed of N_With_Clause node

The Withed_Body field was added to N_With_Clause node to help the
Walk_Library_Items routine, which was created for the CodePeer backend
and later adopted by the GNATprove.

This routine is meant to traverse all library units, such that declarations
are visited before references. However, for complex units (in particular,
with generics and child packages) it never worked reliably and backends
developed their own workarounds. This patch deconstructs the field, as it
hasn't been used for years.

Semantics unaffected; no test provided.

2018-07-16  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* sinfo.ads, sinfo.adb (Withed_Body): Remove.
	(Set_Withed_Body): Remove.

From-SVN: r262721