cleanup: remove obsolete ellipsis complexity

Previously, we chose the ellipsis character/string based on the locale. We now assume a UTF-8 locale, and accordingly always use the Unicode HORIZONTAL ELLIPSIS U+2026 …. When this was changed, some of the logic for handling different ellipsis values was left behind. It no longer serves a purpose, so remove it.

The functions returning constants are replaced by constants. Since the ellipsis as a wstr is only used in a single file, make it a local const there and define it via the ELLIPSIS_CHAR const.

Put the ELLIPSIS_CHAR definition into fish-widestring, removing the dependency of fish-wcstringutil on fish-common, helping future extraction efforts.

One localized message contains an ellipsis, which was inserted via a placeholder, preventing translators from localizing it. Since the ellipsis is a constant, put it directly into the localized string.

Closes #12493