[Ada] Revert change for gnatprove that is no longer needed

[Ada] Revert change for gnatprove that is no longer needed

Years ago, we adapted Get_Kind_Of_Unit so that gnatprove could use it to
detect both bodies and specs of predefined units.

However, this wasn't really needed: gnatprove could simply reuse
Lib.In_Predefined_Unit and now it does. This patch simply reverts two
commits mentioned above.

No frontend test provided, because the removed comment is clear that this
doesn't affect the compiler. Other tools similarly should not be affected
(at least they were not when this change was introduced in 2012).

2019-08-20  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* impunit.adb (Get_Kind_Of_Unit): Revert change for adapting
	this routine for gnatprove.

From-SVN: r274718