ada: Avoid extra conversion in expansion of packed array assignments

Expansion of assignments to packed array objects with string literals on
the right-hand side, created an unnecessary conversion, i.e.:

  ... :=
    component_type
      (declare
         temp : component_type := "string_literal";
       begin
         temp)

Now the expansion omits the outer type conversion.

Cleanup; behavior is unaffected.

gcc/ada/

	* exp_pakd.adb (Expand_Bit_Packed_Element_Set): Simplify handling of
	assignments with string literals.