Remove "-Xcheck:jni" from Java tests (#1402)

Summary:
Junit and our code generate lots of warning if "-Xcheck:jni" is on and force Travis to fail as the logs are too long.

Test Plan: "make jtest" and see the warnings go away.

java/Makefile

@@ -194,7 +194,7 @@ java_test: java resolve_test_deps
 test: java java_test run_test
 
 run_test:
-	java -ea -Xcheck:jni -Djava.library.path=target -cp "$(MAIN_CLASSES):$(TEST_CLASSES):$(JAVA_TESTCLASSPATH):target/*" org.rocksdb.test.RocksJunitRunner $(JAVA_TESTS)
+	java -ea -Djava.library.path=target -cp "$(MAIN_CLASSES):$(TEST_CLASSES):$(JAVA_TESTCLASSPATH):target/*" org.rocksdb.test.RocksJunitRunner $(JAVA_TESTS)
 
 db_bench: java
 	$(AM_V_GEN)mkdir -p $(BENCHMARK_MAIN_CLASSES)