Can we delete confusing feature about inline-config command line option?

**Is your feature request related to a problem? Please describe.**

https://github.com/fluent/fluentd/pull/2708

> I hadn't use this this feature until now.
> I got confused about this feature.

> I expected that inline config is freshly and entirely used for Fluentd configuration.
> However, currently, inline config is appended loaded Fluentd configuration.

> This behavior got confuse for me.

**Describe the solution you'd like**

Just deleting the feature which reads an additional config from STDIN when passing the value `-` as inline-config command-line option to fluentd.

**Describe alternatives you've considered**

**Additional context**

Are there any backgrounds about this feature? @repeatedly