snap: drop notify plugin.

We cant use it on Ubuntu with chromecast enabled, since gtk-3 links to
protobuf.

extras/package/snap/snapcraft.yaml

@@ -103,7 +103,6 @@ parts:
       - libgles2-mesa-dev
       - libgnutls28-dev
       - libgtk2.0-dev
-      - libgtk-3-dev
       - libidn11-dev
       - libiso9660-dev
       - libjack-dev
@@ -118,7 +117,6 @@ parts:
       - libmpeg2-4-dev
       - libmtp-dev
       - libncursesw5-dev
-      - libnotify-dev
       - libogg-dev
       - libomxil-bellagio-dev
       - libopus-dev