Remove unused test function

[peterfpeterson]

Everything has already moved to mantid.testing.assert_not_equal(). This is a maintenance task to remove the (now) unused function.

To test

This is a maintenance that has no functional difference in the code. The builds should pass and nothing else will be affected.

Link to EWM item

EWM#7338

Acceptance Criteria

workspacesNotEqual() should not appear anywhere in the snapred repository.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.76%. Comparing base (32de852) to head (bce79d8).
Report is 2 commits behind head on next.

Additional details and impacted files

@@           Coverage Diff           @@
##             next     #529   +/-   ##
=======================================
  Coverage   95.76%   95.76%           
=======================================
  Files          68       68           
  Lines        5081     5081           
=======================================
  Hits         4866     4866           
  Misses        215      215           

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