Add `-l` option to 'mrbc'.

* The usage of **mrbc**'s `-l` option is little unique. It will **remove** lvar instead of generating it.
* Reverted https://github.com/mruby/mruby/commit/f0b86f36c367e111322aa2f175e05047af29480a since `mrb_irep_remove_lv` will be used in **mrbc**.
* Tests using **lvar** is skipped when `-l` option is enabled in **MRuby::Command::Mrbc**. (To enable `-l` option in `MRuby::Command::Mrbc` add `conf.mrbc.compile_options << ' -l'` to **build_config.rb**.)