Remove/move/document old force field files

[mattwthompson]

There are a handful of files in openff/toolkit/data/test_forcefields that have seem to have been abandoned at some point in time. Since there is only /data/test_forcefields/ and no /data/look_here_for_sample_forcefields, and since this is more or less the canonical user-facing OpenFF repo, it's a a likely path for passersby to stumble into. But,

• Several (i.e. chargeincrement-test.offxml) is not compatible with the current SMIRNOFF spec and/or implementation
• Some (i.e. benzene_minimal.offxml) seem to have been added for testing but are no longer used

• Others aren't listed in the Manifest, so I don't know

  It seems like this folder is a collection of files try to both serve as source data for tests but also as instructive samples ("sample SMIRNOFF ffxml files for different systems/regions of chemical space"). Or at least I find it somewhat confusing what to expect of these files (also probably data/molecules/ but not quite analogous since we're not rolling our own molecule file format). Compare to i.e. evaulator where this distinction is made more clear. If they're test-only, we should remove them or move them to a depurated/ path. If they're user- or dev-facing then there's other considerations.

Also, the current behavior looks into these paths when loading in a force fields; this makes tests a little more succinct but maybe should be removed?

[mattwthompson]

Artificial or not, not yet intelligent enough ...