COMP: Remove breathe link to ConstantPadImageFilter.

and MirrorPadImageFilter.

This does not parse correctly for the "\image" doxygen command that
is contained in the class comments.  For example, for the LaTeX output, it
produces:

   \includegraphics{ITKDoxygenXML/PadImageFilter.png}

Remove until we fix breathe.

Change-Id: I2c25d864c0242283d6aa6ff3de2d71efb70dfed8
Merge "COMP: Remove breathe link to ConstantPadImageFilter."