Add upstream check for correct subs feature

[mjlagattuta]

Observed behaviour

Using the existing glyphs set filters I added .subs glyphs that are identical to inferior glyphs, however in my case they are identical and they are adding unnecessary file size.

Expected behaviour

• [ ] Update the Google Fonts glyph set for Pro removing .subs glyphs
• [ ] Check that the designer has copied the .sinf feature code to the .subs feature code and disabled "automatically generate feature"

Resources and exact process needed to replicate

• See commit on Hepta Slab: https://github.com/mjlagattuta/Hepta-Slab/commit/8dbf26d9ff18e0021a97abf6321472bbbd4ae078
• https://forum.glyphsapp.com/t/how-to-make-subscripts-most-efficiently/8915

[davelab6]

.sinf should be eligible for this too

@m4rc1e assigned this to you

cc @alexeiva