[3161] Removed unread variable.

Found by the clang-static analyzer.

src/lib/util/tests/stopwatch_unittest.cc

@@ -1,4 +1,4 @@
-// Copyright (C) 2015 Internet Systems Consortium, Inc. ("ISC")
+// Copyright (C) 2015-2016 Internet Systems Consortium, Inc. ("ISC")
 //
 // This Source Code Form is subject to the terms of the Mozilla Public
 // License, v. 2.0. If a copy of the MPL was not distributed with this
@@ -246,7 +246,6 @@ TEST_F(StopwatchTest, realTime) {
     usleep(1000);
 
     // The durations should get updated as appropriate.
-    current_duration = stopwatch.getLastMilliseconds();
     EXPECT_GE(stopwatch.getLastMilliseconds(), 1);
     EXPECT_GE(stopwatch.getTotalMilliseconds(), 2);
 }