Another round to remove final modifier to private methods because it is not needed

Remove final modifier to private methods because it is not needed.
Those methods are final by default.

Author: Henry Saputra <henry.saputra@gmail.com>

Closes #168 from hsaputra/remove_final_privatemethods_1 and squashes the following commits:

860b512 [Henry Saputra] Remove final modifier to private methods because it is not needed. It is final by default.
[FLINK-1203] Deactivate fork reuse in tests (workaround for potential surefire bug)

This closes #174
[FLINK-1202] Remove incomplete file outputs on failure

This closes #175