Resolved concurrency issue in POS tagger.

We encountered a concurrency issue in the pos tagger module in the class DefaultPOSContextGenerator.

The issue is demonstrated in DefaultPOSContextGeneratorTest.java. The test "multithreading()" consistently fails on our system with the current code if the number of threads (NUMBER_OF_THREADS) is set to 10. If the number of threads is set to 1 (effectively disabling multithreading), the test consistently passes.

We resolved the issue by removing a field in DefaultPOSContextGenerator.java.