STYLE: Remove alias define with concreate class

COMMON_MMI_METRIC_TYPE  -->  itk::MattesMutualInformationImageToImageMetricv4

There is no need for an alias to the metric type any longer.

Mattes Mutual Informtion does not benefit from more than 3 threads
in most data size cases.  In fact it is detrimental to performance
for huge number of threads.

HACK: Set to maximum number of threads=3 for quick fix.
mutualInformationMetric->SetMaximumNumberOfThreads(3);