ada: Fix spurious -Wstringop-overflow with link time optimization

It comes from an incomplete optimization performed by LTO that is caused by
an obsolete transformation done in Gigi, which is redundant with the common
uniquization of constant CONSTRUCTORs now performed during gimplification.

gcc/ada/

	* gcc-interface/trans.cc (gnat_gimplify_expr) <CALL_EXPR>: Delete.