Remove problem listing of PlainRead which is reworked here

Author: jerboaa

test/hotspot/jtreg/ProblemList.txt

@@ -105,7 +105,6 @@ runtime/ClassInitErrors/TestStackOverflowDuringInit.java 8334545 generic-all
 
 applications/jcstress/copy.java 8229852 linux-all
 
-containers/cgroup/PlainRead.java 8333967,8261242 linux-all
 containers/docker/TestJcmd.java 8278102 linux-all
 containers/docker/TestMemoryAwareness.java 8303470 linux-all
 containers/docker/TestJFREvents.java 8327723 linux-x64