i18n: remove namespace-dependent part from "echo-pref-tooltip-mention"

- The mention is independent from the namespace.

Spotted by JurgenNL
https://translatewiki.net/wiki/Thread:Support/About_MediaWiki:Echo-pref-tooltip-mention/en

Change-Id: I07f828eb1769d52f4a6bf118a34bc5dfbbb313ee
Merge "i18n: remove namespace-dependent part from "echo-pref-tooltip-mention""