[Ada] Use Cancel_Special_Output instead of Set_Special_Output with null

[Ada] Use Cancel_Special_Output instead of Set_Special_Output with null

I believe Cancel_Special_Output is easier to read and thus preferred;
otherwise, it wouldn't be introduced, so let's use it where possible.

2018-11-14  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* bindgen.adb, exp_cg.adb, repinfo.adb, sprint.adb: Minor reuse
	Cancel_Special_Output where possible.

From-SVN: r266110