Remove return_* matcher of command resource

These are obsoleted.

```ruby
describe command('foo') do
  it { should return_stdout /stdout/ }
  it { should return_stderr /stderr/ }
  it { should return_exit_status 0 }
end
```

Instead, you can write like these.

```ruby
describe command('foo') do
  its(:stdout) { should match /stdout/ }
  its(:stderr) { should match /stderr/ }
  its(:exit_status) { should eq 0 }
end
```