Use remote part's implementation of ccache

This patch uses the remote part implementation of ccache to shrink
the lines of code in the recipe.  The implementation of the remote
part can be acquired by running `snapcraft define ccache`.

snap/snapcraft.yaml

@@ -326,40 +326,6 @@ parts:
   ccache:
     after:
       - gcc-6
-    override-pull: 'true'
-    build-packages:
-      - ccache
-    plugin: nil
-    override-build: |
-      mkdir \
-        --parents \
-        "${SNAPCRAFT_PART_INSTALL}"/bin
-      ln \
-        --force \
-        --symbolic \
-        "$(which ccache)" \
-        "${SNAPCRAFT_PART_INSTALL}"/bin/gcc-6
-      ln \
-        --force \
-        --symbolic \
-        "$(which ccache)" \
-        "${SNAPCRAFT_PART_INSTALL}"/bin/g++-6
-      ln \
-        --force \
-        --symbolic \
-        ./gcc-6 \
-        "${SNAPCRAFT_PART_INSTALL}"/bin/gcc
-      ln \
-        --force \
-        --symbolic \
-        ./g++-6 \
-        "${SNAPCRAFT_PART_INSTALL}"/bin/g++
-    filesets:
-      executables:
-        - bin/*
-    stage:
-      - $executables
-    override-prime: 'true'
 
   # This pseudo part setups version 6 of GCC, which is not available
   # in Ubuntu 16.04 software source.