ComputeImageExtremaFilter: Remove `UseMask` property and let it only compute min and max

[N-Dekker]

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No longer computed the mean, sigma, variance, sum, or sum of squares. Removed m_ThreadSum, m_SumOfSquares, and m_Count. Removed inheritance from StatisticsImageFilter (which computed the mean, sigma, variance, sum, and sum of squares for images without a mask).

ComputeImageExtremaFilter is only being used in elastix by AdvancedImageToImageMetric and AdvancedMeanSquaresImageToImageMetric, and both of them only need to have the minimum and the maximum from ComputeImageExtremaFilter.

A performance improvement of ~8 percent was observed on a 8192x8192 image with mask. Without a mask, the computation appears even more than six times as fast as before.