[Ada] Avoid spurious error in GNATprove mode on non-null access types

[Ada] Avoid spurious error in GNATprove mode on non-null access types

GNATprove directly handles non-null access checks, and requires that the
frontend does not insert explicit checks in the form of conditional
exceptions being raised. Now fixed.

There is no impact on compilation.

2019-08-21  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* checks.adb (Install_Null_Excluding_Check): Do not install
	check in GNATprove mode.

From-SVN: r274780