Make fthbfont and derivatives use common HarfBuzz code in hbfont.c

Make fthbfont and derivatives use common HarfBuzz code in hbfont.c

* src/font.h (fthbfont_shape, fthbfont_combining_capability)
[HAVE_HARFBUZZ]: Remove prototypes.

* src/ftfont.c: Don't include math.h.
(uni_combining, uni_general, uni_mirroring, get_hb_unicode_funcs)
(fthbfont_shape_by_hb, fthbfont_combining_capability, fthbfont_shape)
[HAVE_HARFBUZZ]: Remove functions.

* src/ftfont.c (syms_of_ftfont_for_pdumper) [HAVE_HARFBUZZ]:
* src/xftfont.c (syms_of_xftfont_for_pdumper) [HAVE_HARFBUZZ]:
* src/ftcrfont.c (syms_of_ftcrfont_for_pdumper) [HAVE_HARFBUZZ]: Use
hbfont_shape and hbfont_combining_capability instead of fthbfont_shape
and fthbfont_combining_capability, respectively.