Fix main: remove an incompatible java api method with java 8 from a test (#19424)

Looks like the issue with main and the nightly release was caused by the
test in #19354. I checked and that `toList` method does not seem to
affect what is being tested there (the previous error would still appear
without it), so I removed it here.

Author: jchyb

tests/pos/i19354.orig.scala

@@ -10,7 +10,6 @@ class P extends AbstractProcessor {
       .flatMap(annotation => roundEnv.getElementsAnnotatedWith(annotation).stream())
       .filter(element => element.getKind == ElementKind.PACKAGE)
       .map(element => element.asInstanceOf[PackageElement])
-      .toList()
     true
   }
 }