Remove old "itkPCAMetric" files, rename "itkPCAMetric_F_multithreaded" files to "itkPCAMetric"

[N-Dekker]

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The old "itkPCAMetric.h" and "itkPCAMetric.hxx" files were not being used anymore. They were not even #include'd anywhere. Moreover, they cannot even be used anymore, because they would then conflict with "itkPCAMetric_F_multithreaded.h" and "itkPCAMetric_F_multithreaded.hxx", which define a class template with exactly the same name! Using both "itkPCAMetric_F_multithreaded" and "itkPCAMetric" would probably cause link errors, or otherwise maybe undefined behavior.

• Removing these source files eases further maintenance of elastix, including uncoming changes regarding the SubtractMean parameter (pull request #1104)

@mstaring @stefanklein Do you agree that the old "itkPCAMetric.h" and "itkPCAMetric.hxx" files may be removed? And that we can then rename those "itkPCAMetric_F_multithreaded" files to "itkPCAMetric"?

[N-Dekker]

For the record, "itkPCAMetric.h" and "itkPCAMetric.hxx" were unused already with their first commit, 764490d5e37de792fea4340fc11850bfd067318d, "Add: Added PCAMetric, PCAMetric2 and SumOfPairwiseCorrelationCoefficientsMetric...", by Wyke (@whuizinga), 24 Nov 2015. Nevertheless, these source files were maintained with every code modernization and style improvement. (20+ "itkPCAMetric.h" commits, 40 "itkPCAMetric.hxx" commits, and counting...)

[N-Dekker]

Discussed at the weekly internal elastix sprint. Marius and Stefan agreed that it is OK to remove the old "itkPCAMetric" files, and rename the "itkPCAMetric_F_multithreaded" files to "itkPCAMetric".