As part of the work on changing side-effects removal in SPARK, a special
case was introduced to generate an Itype_Reference for Itypes in slices.
This was based on a misunderstanding of existing checks for bounds when
analyzing slices. These Itype_Reference are actually not needed to get
the corresponding run-time checks in GNATprove, and are actually harmful
in some cases (inside quantified expressions) as the insertion point for
the Itype_Reference ends up being outside of the quantifier scope,
leading to unprovable checks.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* sem_res.adb (Set_Slice_Subtype): Revert special-case
	introduced previously, which is not needed as Itypes created for
	slices are precisely always used.