remove legacy linux distro install docs

Author: leafo

SUMMARY.md

@@ -6,10 +6,6 @@
     * [Installing on Windows](installing/windows.md)
     * [Installing on macOS](installing/macos.md)
     * [Installing on Linux](installing/linux/README.md)
-      * [Installing on Ubuntu & Debian](installing/linux/ubuntu-and-debian.md)
-      * [Installing on Fedora](installing/linux/fedora.md)
-      * [Installing on ArchLinux](installing/linux/archlinux.md)
-      * [Installing on Gentoo](installing/linux/gentoo.md)
   * [Logging in](using/login.md)
     * [Using the app offline](using/offline.md)
   * [Preferences](using/preferences.md)