[cff] Remove MM/Var support
CFF outlines are not affected by OpenType variations and fonts in CFF format (not Type 1) with Multiple Masters seem exceedingly rare. The existing support re-uses the TrueType implementations which works because CFF_Face is the same as TT_Face. However, this runs into issue with CFF_Size not being the same as a TT_Size when the TrueType code assumes that TT_Face implies TT_Size.
Fixes: https://bugs.chromium.org/p/chromium/issues/detail?id=1429372