Remove env_var: and command_line: from Options

[wks]

We used to have some items in mmtk::util::options::Options that can only be set via environment variables but not the command line, or vice versa. One example is MMTK_THREADS due to MMTk being initialized via lazy_static!. After we introduced MMTKBuilder, this restriction is removed.

Now all options can be set via both environment variables and command line. We may remove the redundant [env_var: true, command_line: true] from the options declaration. e.g. What we have now

    plan:                  PlanSelector         [env_var: true, command_line: true] [always_valid] = PlanSelector::GenImmix,
    threads:               usize                [env_var: true, command_line: true] [|v: &usize| *v > 0]    = num_cpus::get(),

will become

    plan:                  PlanSelector         [always_valid] = PlanSelector::GenImmix,
    threads:               usize                [|v: &usize| *v > 0]    = num_cpus::get(),