[Flutter] Don't define SK_GL by default for Flutter.
[Flutter] Don't define SK_GL by default for Flutter.

The iOS target in Flutter is getting rid of OpenGL altogether. This define
gets in the way of setting `skia_use_gl = False`.

Change-Id: Ifde337c72395ed88c4e94dc3cd33f110a0db5a6d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/562341
Reviewed-by: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>
Commit-Queue: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>

google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia