BUG: Remove 6 unused orphaned ExternalData tags

Six .md5 tag files point to MD5 hashes that 404 on gh-pages, Girder,
and itk.org. None of them are referenced by an active test.

Baselines (4): itkFrequencyExpand{Even,Odd}Test.tiff.md5 and
itkFrequencyShrink{Even,Odd}Test.tiff.md5. Their --compare references
in test/CMakeLists.txt are already commented out.

Inputs (2): checkershadow_gray_540x420.jpg.md5 and
checkershadow_gray_540x420.tiff.md5. Neither is referenced anywhere
in test/CMakeLists.txt.

A 7th orphan tag remains in this module
(test/Input/checkershadow_Lch_512x512.tiff.md5) which 10 active
tests depend on; disabling those tests would gut a large portion
of the test suite, so it is left for separate discussion.

Author: hjmjohnson