Mullapudi2016: don't hardcode the list of supported targets (#6520)

As discussed in #6518,
this is a bit dubious, and e.g. prevents building on RISC-V,
because there is no way to not build autoschedulers currently.

src/autoschedulers/mullapudi2016/AutoSchedule.cpp

@@ -3377,9 +3377,6 @@ struct Mullapudi2016 {
  results.target = target;
  results.machine_params_string = arch_params.to_string();
 
- user_assert(target.arch == Target::X86 || target.arch == Target::ARM ||
- target.arch == Target::POWERPC || target.arch == Target::MIPS)
- << "The Mullapudi2016 autoscheduler is not supported for the target: " << target.to_string();
  results.scheduler_name = "Mullapudi2016";
  std::vector<Function> pipeline_outputs;
  for (const Func &f : pipeline.outputs()) {