remove custom fonts, instead use client's font:message-box

Rather than serving downloadable fonts (Clear Sans, Fira Sans) for the pages here, we should use the client's default user-interface font (a) to better match the platform's expected appearance for user interface elements (which is conceptually what these pages are, despite being implemented as content), and (b) for better i18n-readiness (e.g. see issue 860; note that Turkish is only one example of languages that are not well supported by the current fonts). See also https://bugzilla.mozilla.org/show_bug.cgi?id=992650.