[googlefonts/metadata/valid_filename_values] appears redundant

[simoncozens]

This check ensures that a font's filename appears in the filename field of any of the fonts entries in the metadata file, and fails if the filename is not found.

But googlefonts/metadata/filenames also looks at all the font filenames in the current directory, and checks that they appear in the filename field of a font entry in the metadata file, failing if the filename is not found.