[Ada] Fix uses of pragma Unreferenced in MinGW runtime unit

[Pierre-Marie de Rodat]

GNAT now emits more warnings about pragma Unreferenced.

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

gcc/ada/

	* libgnarl/s-taprop__mingw.adb (Timed_Sleep): Remove "pragma
	Unreferenced" for Result.
	(Timed_Delay): Likewise.
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