[Ada] Remove dead code from Enclosing_Package_Or_Subprogram routine

[Ada] Remove dead code from Enclosing_Package_Or_Subprogram routine

Calls to Scope always return unique entities, i.e. package/subprogram
and not their bodies, so there is no need to expect them.

Cleanup only; semantics unaffected. (This routine was only used in CCG
and GNATprove backends anyway.)

2019-07-08  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* sem_util.adb (Enclosing_Package_Or_Subprogram): Do not expect
	package and subprogram bodies.

From-SVN: r273222