[gnome-clocks] snap: stop using core18

[Jeremy Bicha <jbicha src gnome org>]

commit ea16a0a3c1b4f145235c6de18595de983d7cb9c3
Author: Jeremy Bicha <jbicha ubuntu com>
Date:   Thu Jan 31 18:57:02 2019 -0500

    snap: stop using core18
    
    sound isn't working
    
    org.gnome.Clocks-WARNING **: utils.vala:335: Error playing sound: No such driver

 snap/snapcraft.yaml | 15 +--------------
 1 file changed, 1 insertion(+), 14 deletions(-)
---
diff --git a/snap/snapcraft.yaml b/snap/snapcraft.yaml
index 554902b..770c84d 100644
--- a/snap/snapcraft.yaml
+++ b/snap/snapcraft.yaml
@@ -7,7 +7,6 @@ description: |
 
 grade: stable # must be 'stable' to release into candidate/stable channels
 confinement: strict
-base: core18
 
 # Launchpad doesn't recognize these fields yet
 passthrough:
@@ -15,7 +14,7 @@ passthrough:
   title: GNOME Clocks
 
 plugs:
-  gnome-3-28-1804:
+  gnome-3-26-1604:
     interface: content
     target: $SNAP/gnome-platform
     default-provider: gnome-3-26-1604
@@ -56,18 +55,6 @@ apps:
       LD_LIBRARY_PATH: $LD_LIBRARY_PATH:$SNAP/lib/gnome-clocks
 
 parts:
-  desktop-gnome-platform:
-    source: https://github.com/ubuntu/snapcraft-desktop-helpers.git
-    source-subdir: gtk
-    plugin: make
-    make-parameters: ["FLAVOR=gtk3"]
-    build-packages:
-      - build-essential
-      - libgtk-3-dev
-    override-build: |
-      snapcraftctl build
-      mkdir -pv $SNAPCRAFT_PART_INSTALL/gnome-platform
-
   libgweather:
     source: https://git.gnome.org/browse/libgweather
     source-type: git