Remove local type aliases of IteratorWithIndex in *.hxx files

[N-Dekker]

Made it easier to figure out when an iterator "with index" is being used.

Using Notepad++, Replace in Files, doing:

Find what: ^[ ]+ using (\w+) = (Image\w+IteratorWithIndex<\w+>);\r\n(.*?[^\w])\1
Replace with: $3$2
(*) Regular expression
[v] . matches newline

BinomialBlurImageFilter and CheckerBoardImageFilter were adjusted manually.

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Note: it appears that some filters accidentally use an iterator "with index", while they should have used a different iterator type, for performance reasons. Such possible mistakes should be easier to spot when it is directly obvious from the iterator variable declarations that those iterators are "with index".

When reviewing, it might be helpful to ignore whitespace changes: https://github.com/InsightSoftwareConsortium/ITK/pull/5839/changes?w=1

[dzenanz]

Mild approval. Reservation: this sometimes makes the iterator declaration span multiple lines.

[N-Dekker]

Mild approval. Reservation: this sometimes makes the iterator declaration span multiple lines.

Thanks @dzenanz As far as I can see, it only happens a few times that these iterator declaration span multiple lines. Most cases still fit on a line.

Other mitigating factors:

• We did already shorten various iterator declarations very recently by pull request Remove itk::, when using Iterator type in itk namespace #5818
• We could reconsider using CTAD (class template argument deduction) for "WithIndex" variables, as a follow-up PR, so that we don't need to explicitly specify the template argument (usually TInputImage or TOutputImage) with each variable declaration.
• It appears that some of the iterators don't really need to be "with index". Removing WithIndex in those cases will shorten those iterator variable declaration.