snap: Skip installing postcss due to failure on build server

npm running on fails to fetch postcss-cli on build.snapcraft.io
due to network proxy issue.

See https://forum.snapcraft.io/t/build-fails-behind-proxy-on-build-snapcraft-io/6951/3

snapcraft.yaml

@@ -58,16 +58,8 @@ parts:
 
   node:
     plugin: nodejs
-    node-packages: [postcss-cli]
-    filesets:
-      node:
-        - bin/node
-      postcss:
-        - bin/postcss
-        - lib/node_modules/postcss-cli/*
     prime:
-      - $node
+      - bin/node
-      - $postcss
 
   pygments:
     plugin: python