[Ada] Remove External aspect from predefined abstract states

Explicit External aspect was an equivalant to an implicit default.  It was only
needed as a workaround for a frontend bug. (If it meant to serve as
documentation, there should be explicit Effective_Reads and Effective_Writes
set to False too.)

No test, because these changes are semantically neutral.

2018-05-21  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* libgnarl/a-reatim.ads (Clock_Time): Remove External aspect.
	* libgnarl/a-taside.ads (Tasking_State): Likewise.
	* libgnat/a-calend.ads (Clock_Time): Likewise.

From-SVN: r260455

gcc/ada/ChangeLog

+2018-04-04  Piotr Trojanek  <trojanek@adacore.com>
+
+	* libgnarl/a-reatim.ads (Clock_Time): Remove External aspect.
+	* libgnarl/a-taside.ads (Tasking_State): Likewise.
+	* libgnat/a-calend.ads (Clock_Time): Likewise.
+
 2018-04-04  Yannick Moy  <moy@adacore.com>
 
 	* sem_ch6.adb (Analyze_Procedure_Call): Refine test to recognize prefix

gcc/ada/libgnarl/a-reatim.ads

@@ -38,9 +38,7 @@ pragma Elaborate_All (System.Task_Primitives.Operations);
 
 package Ada.Real_Time with
   SPARK_Mode,
-  Abstract_State => (Clock_Time with Synchronous,
-                                     External => (Async_Readers,
-                                                  Async_Writers)),
+  Abstract_State => (Clock_Time with Synchronous),
   Initializes    => Clock_Time
 is
 

gcc/ada/libgnarl/a-taside.ads

@@ -38,9 +38,7 @@ with System.Tasking;
 
 package Ada.Task_Identification with
   SPARK_Mode,
-  Abstract_State => (Tasking_State with Synchronous,
-                                        External => (Async_Readers,
-                                                     Async_Writers)),
+  Abstract_State => (Tasking_State with Synchronous),
   Initializes    => Tasking_State
 is
    pragma Preelaborate;

gcc/ada/libgnat/a-calend.ads

@@ -35,9 +35,7 @@
 
 package Ada.Calendar with
   SPARK_Mode,
-  Abstract_State => (Clock_Time with Synchronous,
-                                     External => (Async_Readers,
-                                                  Async_Writers)),
+  Abstract_State => (Clock_Time with Synchronous),
   Initializes    => Clock_Time
 is