Remove the loader constructor that swallowed init's error

loader(buf, buf_size, callbacks) called init() and discarded the
loader_error, leaving a silently half-initialized object with no way
for the caller to tell. PawnIO does not use it -- vm_load_binary_internal
default-constructs and then checks init()'s result -- so delete it
rather than invent a way to retrieve the status after the fact.