testsuite: Remove unused dg-line in ce8cdf5b

Test case g++.dg/analyzer/fanalyzer-show-events-in-system-headers.C
introduced by patch ce8cdf5b
emitted a warning for an unused dg-line variable.
This fixes up the blunder.

Signed-off-by: benjamin priour <vultkayn@gcc.gnu.org>

gcc/testsuite/ChangeLog:

	* g++.dg/analyzer/fanalyzer-show-events-in-system-headers.C:
	Remove dg-line var declare_a.