[#3575358]libfm-pref-apps: set browser as default for https too.

Every decent browser supports both http and https so there is no reason
to set default application only for http.
Remove writing ~/.config/libfm/pref-apps.conf because it duplicates XDG.

Since libfm-pref-apps already uses XDG calls all data are written
into standard mimeapps.list file and using pref-apps.conf is never
required.