[Ada] Disable unwanted warnings in Assertion_Policy(Ignore) mode

[Ada] Disable unwanted warnings in Assertion_Policy(Ignore) mode

2020-06-10  Bob Duff  <duff@adacore.com>

gcc/ada/

	* sem_prag.adb (Invariant): Remove the pragma removing code.  It
	doesn't work to remove the pragma, because various flags are set
	during Build_Invariant_Procedure_Declaration and
	Build_Invariant_Procedure_Body that need to be set to avoid the
	spurious warnings.
	* exp_util.adb (Make_Invariant_Call): Avoid calling the
	invariant-checking procedure if the body is empty. This is an
	optimization.