Delete duplicate math-holds-true statement.

The statement was duplicated (see https://github.com/A1kmm/cellml-core-spec/commit/24fadcf#diff-fd9436c7664b85ca3fd96c4fd7f7893bL1080), and then only one of the duplicates later edited.
Merge pull request #9 from codecurve/deleteDuplicate-MathHoldsTrue

Delete duplicate math-holds-true statement.