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
Merge "Remove Python 2.6 backwards compatibility code"