[ft] Don't set font ppem

For discussion see:

  http://lists.freedesktop.org/archives/harfbuzz/2012-April/001905.html

Over time we have had added NO_HINTING all over the place in hb-ft.  Finish it off.
Not setting ppem on hb-font disables get_contour_point() calls which is good anyway.

See comments in the commit.