mruby-task: remove unnecessary mruby-fiber dependency

mruby-task uses mrb_context and mrb_fiber_state enum, but these are
part of core mruby, not the mruby-fiber gem. the dependency was not
needed.

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-task/mrbgem.rake

@@ -10,6 +10,4 @@ MRuby::Gem::Specification.new('mruby-task') do |spec|
   if spec.for_windows?
     spec.linker.libraries << 'winmm'
   end
-
-  spec.add_dependency 'mruby-fiber'  # Uses same context infrastructure
 end