Get rid of unused env variable

TEMPEST_LOG_CONFIG was never interpreted within tempest.
As a result logging.conf was only interpreted if log_config_append
in tempest.conf was set manually.

Logging properties are important for tempest CLI to ensure
the user sees some output.

Change-Id: I2bcf386b9dd2afd78ed81fd62fcc54e9b63ddeab
Partial-Bug: #1527142
2 files changed