Remove the note about x11-fonts/mathfonts, which has been removed.

According to http://www.mozilla.org/projects/mathml/fonts/ many other
fonts can be used, e.g. the STIX fonts.

PR: ports/144124
Submitted by: Tsurutani Naoki <turutani (at) scphys.kyoto-u.ac.jp>
Feature safe: yes