feature.h - simplify expression CURRENT_HINTS expression

feature.h - simplify expression CURRENT_HINTS expression

PL_hints expands to PL_compiling.cop_hints. So there is no
point in doing

    PL_curcop == &PL_compiling ? PL_hints : PL_curcop->cop_hints

as it resolves to doing:

    PL_curcop->cop_hints

but with a comparison, which likely kills any speed advantage
the direct access might provide.

The old define is left around to validate that the two are the same
if needed.