| Elektra 0.11.0

Use Case: Remove `Key` from `KeySet`

# Summary

- Scope: `libelektra-core`
- Level: Developer Goal
- Actors: Core, Caller
- Brief: Caller removes `Key` from existing `KeySet`

# Scenarios

- Precondition:
  - `KeySet` has been created
- Main success scenario:
  - Caller requests to remove `Key` by name from `KeySet`
  - Core searches for `Key` with same name in `KeySet`
  - If a matching `Key` is found, Core removes it from `KeySet` returns a `Key *` to it. The name of the `Key` will be read-only, otherwise it is modifiable.
  - Otherwise, Core returns `NULL`
- Alternative scenario: -
- Error scenario: -
- Postcondition:
  - The removed `Key`(s) MUST NOT be part of `KeySet` anymore. A lookup for that/those `Key`s MUST NOT return any results.
- Non-functional Constraints: -