Remove cruft from LanguageNegotiationMethodManager

Problem/Motivation

LanguageNegotiationMethodManager could call setCacheBackend() instead of initialising the properties itself. In fact it looks like the $cacheKeyPrefix is not even used so enabling this has caught some historical cruft.

@longwave in #3377000-4: [PP-1] Remove phpstan ignore "#^Missing cache backend declaration for performance.#"

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet