Remove Python 2.6 backwards compatibility code

Remove Python 2.6 backwards compatibility code

Python 2.6 is no longer supported in cinder as of kilo. The code in the
project that is specifically for compatibility with 2.6 is therefore no
longer required.

This commit removes code referenced as being required specifically for
compatibility with Python 2.6.

This commit removes:
    - total_seconds from cinder/utils.py
    - TestCase.assertGreater from cinder/test.py
    - TestCase.assertGreaterEqual from cinder/test.py
    - StorwizeSVCDriverTestCase.assertLessEqual from
      cinder/tests/test_storwize_svc.py

Change-Id: I2aca4a6a84bc8ddfa70bd47a331b6fac6f82220f