snap: Remove unused permissions and duplicated libraries (#2478)

* snap: Remove various unneeeded plugs

Remove various permissions that the snap required and that are not
actually used. In particular now all the file and settings access is handled
by XDG portals, so there's no need to grant direct file access.

Also this is not a media app or network management software, so no
need to add screen inhibitors or network observation.

* snap: Do not add libnss3

That's already provided by GNOME so no need to duplicate it.

* Update snapcraft.yaml

Put back password-manager-service, home for now

---------

Co-authored-by: Ben Gotow <bengotow@gmail.com>

snap/snapcraft.yaml

@@ -26,7 +26,6 @@ parts:
     parse-info: [usr/share/appdata/mailspring.appdata.xml]
     stage-packages:
       - libxkbfile1
-      - libnss3
     stage:
       - -usr/lib/*/libsecret-*
 
@@ -60,18 +59,13 @@ apps:
       - avahi-observe
       - browser-support
       - cups-control
-      - gsettings
       - opengl
       - home
       - desktop
       - desktop-legacy
-      - mount-observe
       - network
       - network-bind
-      - network-observe
       - password-manager-service
       - pulseaudio
-      - removable-media
-      - screen-inhibit-control
       - unity7
       - x11