Remove ITK_EXPORT

Summary

Remove ITK_EXPORT.

Rationale

Does nothing since ITK 4.5: https://github.com/InsightSoftwareConsortium/ITK/commit/b788601abbeebd72e5504b10a75a9f417f666c35

From itkMacro.h:

// For backwards compatibility define ITK_EXPORT to nothing
#define ITK_EXPORT

Copyright

The copyright owner is CNES and has signed the ORFEO ToolBox Contributor License Agreement.

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Check before merging:

• All discussions are resolved
• At least 2 👍 votes from core developers, no 👎 vote.
• The feature branch is (reasonably) up-to-date with the base branch
• Dashboard is green
• Copyright owner has signed the ORFEO ToolBox Contributor License Agreement