[truetype] Remove `TT_CONFIG_OPTION_NO_BORING_EXPANSION`.

Despite its name, this option only gated 'avar' version 2 support.
FreeType 2.15.0 includes the complete boring-expansion set of proposals,
now released as ISO Open Font Format, fifth edition
(ISO/IEC 14496-22:2026).  Gating just avar2 no longer makes sense,
especially since it has also been shipping on Apple platforms for
four years.

* include/freetype/config/ftoption.h, devel/ftoption.h
  (TT_CONFIG_OPTION_NO_BORING_EXPANSION): Remove.
* src/truetype/ttgxvar.c (ft_var_load_avar): Remove configuration guards.

* docs/CHANGES: Updated.

All five Meson regression tests pass with the removed macro still
defined.  avar1 and avar2 coordinate mapping agrees with FontTools.

Assisted-by: OpenAI Codex