COMP: On MacOSX, do not create duplicate of MRMLIDIOPlugin and MGHIOPlugin

This fix MacOSX warning "/path/to/plugin is already loaded"

Both MRMLIDIOPlugin and MGHIOPlugin were copied so that it exists both
a "dylib" and a "so" version of the file. This was initially committed
on the 2007-09-27 to work around a limitation of ITK preventing it from
loading plugin having ".dylib" extension. See Slicer3 r4454 [1]

Then 2 months later, on the 2007-11-19, a fix has been pushed in ITK so
that plugin having extension "dylib" are considered. See ITK commit 28b4a6ba [2]

This commit account for the ITK fix by removing the code in charge of
copying the plugin.

Lessons learned: When implementing a hack, it is a good practice to report
a bug upstream to track the limitation and also reference the bug in a
comment starting with either "# HACK" for CMake file or "// HACK" for
c/cpp files. It would give a hint to developer re-factoring the code.

[1] http://viewvc.slicer.org/viewvc.cgi/Slicer3?view=revision&revision=4454
[2] https://github.com/Kitware/ITK/commit/28b4a6ba
ENH: Move MacOSX configuration code for CLI factory to a better place.

As a slight improvement, the path is also appended only if it exists.
ENH: Simplify logic by appending path to CLI factory only if they exist.
ENH: work in prog