Delete duplicate math-holds-true statement.

The statement was duplicated (see A1kmm/cellml-core-spec@24fadcf#diff-fd9436c7664b85ca3fd96c4fd7f7893bL1080 or https://github.com/codecurve/cellml-core-spec/commit/24fad#diff-fd9436c7664b85ca3fd96c4fd7f7893bR1155), and then only one of the duplicates later edited.