(mu-cite-save-registration-file): Use `save-buffer' instead of

(mu-cite-save-registration-file): Use `save-buffer' instead of
`save-buffer-as-coding-system' if the value of
`mu-cite-registration-file-coding-system-for-write' is nil.
(mu-cite-load-registration-file): Use `insert-file-contents' instead of
`insert-file-contents-as-coding-system' if the value of
`mu-cite-registration-file-coding-system-for-read' is nil.
(TopLevel): Don't warn for the obsolete variables if the value of
`noninteractive' is non-nil.
(mu-cite-registration-file): Likewise.