Consistently respect `only_type` and `only_field_*` params

This fixes around a bug in `BaseComponent.prototype.loadConfig` where
your config won't processed unless your filter has some specific
configuration.

`only_type` and `only_field_* params are implicitly processed by
`BaseComponent.prototype.loadConfig`, but `loadConfig` has a "do we
need to load config" check at the top of the function that might fail,
at which point the implicit options is ignored.

For example, a custom filter
`filter://my_thing://only_type=syslog`. `my_thing` doesn't need any
other parameters, so it's constructor looks like

```js
function FilterMyThing() {
  base_filter.BaseFilter.call(this);
  this.mergeConfig({
    name: 'MyThing'
  });
}
```

In the previous implementation, `BaseComponent.prototype.loadConfig`
will decide no config needs to be loaded, ignore the `only_type`
param, and `my_thing` filter will apply to all messages.

This patch removes the if statement, and re-indents. The `loadConfig`
function is only called once when the filter is loaded, so any
performance penalty by processing all the options is a one-time cost.