[TASK] Remove inaccessible code from ArrayConverter
Remove code that is currently inaccessible, because the constant
STRING_FORMAT_SERIALIZED is not defined in the class.
Unserializing from untrusted sources should not be done anyway
so we remove this possibility completely instead of adding the constant.
A use case which would require an unserialize for array conversion is complex
enough to be handled in a dedicated type converter class which exactly fits the use case
instead of providing a potentially insecure shortcut for that (unserialize) in the framework.
Releases: master, 2.3, 2.2