Add `Igniter.Project.Config.remove_configuration`

**Is your feature request related to a problem? Please describe.**
Currently, it is possible to configure keys, but not possible to remove existing configured keys.

**Describe the solution you'd like**
`Config.remove_configuration` or `Config.remove_key` or something like that.

**Describe alternatives you've considered**
It's currently possible to run `Config.configure/6` and pass `nil` or `[]` or `Keyword.remove_keyword_key/2` to clear the configuration, but it would be ideal if there's a dedicated function to remove configuration instead, which should also remove the entire `config :key ...` line as well if everything became empty.

Start by locating the Config.configure/6 implementation and the code that edits existing `config :key ...` entries. Compare how `nil`, `[]`, and `Keyword.remove_keyword_key/2` affect configuration, then verify that the new removal operation clears the requested key and removes the entire config line when no entries remain.

Written by the indexing model from the issue text.