[Ada] Remove External aspect from predefined abstract states

[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