don't require openTypeVheaCaret(Offset|Slope) info attributes to build vhea/vmtx
don't require openTypeVheaCaret(Offset|Slope) info attributes to build vhea/vmtx

the current default fallback values in fontInfoData are good enough:

    openTypeVheaCaretSlopeRise=0
    openTypeVheaCaretSlopeRun=1
    openTypeVheaCaretOffset=0

Fixes https://github.com/googlefonts/fontmake/issues/586

jamesgk/ufo2fdk,googlefonts/ufo2ft,googlei18n/ufo2ft,jamesgk/ufo2ft,moyogo/ufo2ft