Remove references to non-existing message keys.
Triggered on every startup module request:
> [resourceloader] WARNING:
> Failed to find multimediaviewer-option-header-viewer ({lang})
* multimediaviewer-option-header-viewer
(listed in 1853540cd3, but never created)
* multimediaviewer-option-header-filepage
* multimediaviewer-option-desc-viewer
* multimediaviewer-option-desc-filepage
* multimediaviewer-license-cc-by-sa-3.0-migrated
(CommonsMetaData has this aliased to "cc-by-sa-3.0")
* multimediaviewer-description-page-popup-text-local
Even if JS tries to use them, the messages do not exist. There
are no matches across all Wikimedia Git repositories for these
strings other than extension.json here (no i18n, JS or PHP files).
Bug: T118820
Change-Id: I3585f4862781799d9ba828bdc4170a68c6c36100