Remove autohint tool

[miguelsousa]

Remove autohint tool in favor of psautohint.

• add tests to validate that psautohint produces the same output as autohint. This will require some adjustment to autohint and test code:
  • update autohint to output PFB when PFB is supplied as input (currently outputs PFA). See f25415a
  • add method in test_utils.py to use fontTools.cffLib.specializer, to enable comparison of equivalent Type2 Charstrings in CFF-based output (currently, output from autohint is optimized, while output from psautohint is not).
  • skip comparisons where non-default-layer UFO2 is used, since psautohint can only write to the default layer for UFO2.
  • fix bug in autohint where it does not delete com.adobe.processedgyphs layer if present when run with the -wd option. Fix should probably go here: https://github.com/adobe-type-tools/afdko/blob/develop/python/afdko/autohint.py#L1036
• replace content in autohint.py that simply calls psautohint
• add FutureWarnings in above
• Remove autohint.py and related test code from AFDKO [early 2020]