Inherit XeTeXFontInst_Mac from XeTeXFontInst_FT2

So that we can share the FreeType code especially for HarfBuzz, right
now on Mac HarfBuzz uses FreeType when fonts are loaded by file name and
our Core Text-based font functions when fonts are loaded by font name,
which is not good.
Remove now unused HarfBuzz font functions
Use XeTeXFontInst_FT2 for those