Revert "Update snapcraft.yaml to bundle the gnome-3-34 extension"

[ma8ma]

Reverts #378

gnome-3-34 extensionを使ったビルドに失敗したためコミットを取り消します。

• amd64: https://build.snapcraft.io/user/JDimproved/JDim/1027980

  libtool: Version mismatch error.  This is libtool 2.4.6.44-b9b4, but the
  libtool: definition of this LT_INIT comes from libtool 2.4.6.
  libtool: You should recreate aclocal.m4 with macros from libtool 2.4.6.44-b9b4
  libtool: and run autoconf again.
  Makefile:588: recipe for target 'jdim' failed
  

• i386: https://build.snapcraft.io/user/JDimproved/JDim/1027979

  Could not install snap defined in plug 'gnome-3-34-1804'.
  The missing library report may have false positives listed if those libraries are provided by the content snap.
  

• ppc64el: https://build.snapcraft.io/user/JDimproved/JDim/1027983

  Failed to install or refresh a snap: 'gnome-3-34-1804-sdk' does not exist or is not available on the desired channel 'latest/stable'.
  Use `snap info gnome-3-34-1804-sdk` to get a list of channels the snap is available on.
  

• s390x: https://build.snapcraft.io/user/JDimproved/JDim/1027984

  Failed to install or refresh a snap: 'gnome-3-34-1804-sdk' does not exist or is not available on the desired channel 'latest/stable'.
  Use `snap info gnome-3-34-1804-sdk` to get a list of channels the snap is available on.
  

関連のissue: #314