Remove unused junit runners from test-utils module

The two classes under https://github.com/apache/hive/tree/master/testutils/src/java/org/apache/hive/testutils/junit/runners namely:

• ConcurrentTestRunner
• ConcurrentScheduler

have been introduced a long time ago by HIVE-2935 to somewhat parallelize execution for TestBeeLineDriver.

However, since HIVE-14444 (resolved 6 years ago) they are not used by anyone and unlikely to be used again in the future since there are much more modern alternatives.