We like check_struct() more.

ArrayOf<>, unlike UnsizedArrayOf<>, has data before the array.
This was confusing.  Remove.

It wasn't doing anything.

Untested.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1160

https://github.com/harfbuzz/harfbuzz/issues/1342#issuecomment-434829028

Fixes https://github.com/harfbuzz/harfbuzz/issues/1342

Fixes https://github.com/harfbuzz/harfbuzz/issues/1343

It's true that blob->as<> should only be called on null or sanitized
data.  But this change is safe, so keep it.

https://github.com/harfbuzz/harfbuzz/pull/1334

 ./hb-shape Apple_Chancery_10_12.ttf  "Th th ll te to tr fr fu fj"
[T_h=0+2308|space=2+569|t_h=3+1687|space=5+569|l_l=6+1108|space=8+569|t_e=9+1408|space=11+569|t_o=12+1531|space=14+569|t_r=15+1385|space=17+569|f_r=18+1432|space=20+569|f_u=21+1733|space=23+569|f_j=24+1098]

Part of https://github.com/harfbuzz/harfbuzz/issues/1331