Remove documentation that recommends XEmacs It has been nearly a decade since the last stable release of XEmacs, and editors/xemacs was removed from the ports tree in 2014.

[jrm]

Test Plan

cd en_US.ISO8859-1/books/fdp-primer && make FORMATS=html
passes

cd en_US.ISO8859-1/htdocs && make

(succeeds until it tries to reference translations, which I don't have in
my partial checkout)

cd: /usr/home/jrm/scm/freebsd/doc/head/en_US.ISO8859-1/htdocs/../../da_DK.ISO8859-1/htdocs: No such file or directory