Clean up DiscoveryTraitTest::testDoGetDefinitionException

Follow-up #2822837: Replace @expectedException @expectedExceptionMessage with $this->setExpectedException:

@klausi #77:

+++ b/core/tests/Drupal/Tests/Component/Plugin/Discovery/DiscoveryTraitTest.php
@@ -58,7 +59,6 @@ public function providerDoGetDefinitionException() {
@@ -69,6 +69,7 @@ public function testDoGetDefinitionException($expected, $definitions, $plugin_id

@@ -69,6 +69,7 @@ public function testDoGetDefinitionException($expected, $definitions, $plugin_id
     $method_ref = new \ReflectionMethod($trait, 'doGetDefinition');
     $method_ref->setAccessible(TRUE);
     // Call doGetDefinition, with $exception_on_invalid always TRUE.
+    $this->setExpectedException(PluginNotFoundException::class);
     $this->assertSame(
       $expected,
       $method_ref->invoke($trait, $definitions, $plugin_id, TRUE)

It doesn't really make sense to use assertSame() here. The @dataProvider on that method also does not make sense because it has only one test set.