Remove unused reference

Changed files
-1
doc
-1
doc/manpage-urls.json
···
"mpd(1)": "https://mpd.readthedocs.io/en/latest/mpd.1.html",
"mpd.conf(5)": "https://mpd.readthedocs.io/en/latest/mpd.conf.5.html",
"nix.conf(5)": "https://nixos.org/manual/nix/stable/command-ref/conf-file.html",
-
"mount(5)": "https://linux.die.net/man/8/mount",
"journald.conf(5)": "https://www.freedesktop.org/software/systemd/man/journald.conf.html",
"logind.conf(5)": "https://www.freedesktop.org/software/systemd/man/logind.conf.html",