We might reintroduce it later, but for now remove, as it was unused.
Some things that should have been in this object (drop_hints, etc)
are already in hb_subset_input_t. So, for now, keep everything there.
Include stdbool.h in hb-setset-test.h instead of in the individual
sources, if it is found; otherwise use a simplistic fallback for it if
it is not found.
Also declare variables at the top of the block, to build on pre-C99
compiliers.
