[3.x] Remove the support for no-args methods annotated with `@Inject`

[As discussed here](https://github.com/apache/logging-log4j2/pull/3877#discussion_r2284431460), DI in `3.x` supports no-args methods annotated with `@Inject` – yet there are no use cases, not even one we make use of ourselves. I suggest removing this unused feature to avoid an API bloat. We can always consider adding it back if need arises.