babl: drop long return from conversion funcs

babl: drop long return from conversion funcs

They are propagated and instructions are spent on that, but their value are
never used. Some conversion functions in base even used the argument passed for
counting down to 0 and return 0 instead, to no ill effect.

Since we do not currently see external babl extensions, we can break extensions
API without bumping the library version, this is even a change where due to
varargs, old extensions will continue working until migrated.