Remove coretext_aat shaper (#1581)

Remove coretext_aat shaper (#1581)

coretext_aat was a temporary shaper to redirect shaping of AAT fonts
to CoreText and leaving the rest for HarfBuzz.  As HarfBuzz now supports
AAT and Chrome now actually ships that on a stable version on macOS,
we no longer care about such use-case.  If a client really wants 100%
metrics compatibility with CoreText better to use it directly or through
our API.  Replicating the same behavior still is possible using
hb_shape_full, something we don't care or like to offer anymore.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1478