Remove redundant steps in compatibility tests: verifyReadback() method

Several tests including ManagerCompatTest do the following:

1) Tests insert data
2) Compare output from the db against inserted values
3) Run Sqoop import jobs
4) Compare output read from seq files against inserted values

Tthe step 2 is redundant since the step 4 also verifies output. In addition, these tests are fragile since output formats vary depending on versions/vendors of the DB/JDBC driver.

Since there is little value to gain from these tests while they cause compatibility problems, it would be better to remove them.