Depreciate and remove dokku backup plugin, replace with documentation

plugins/git/backup-import

@@ -1,20 +0,0 @@
-#!/usr/bin/env bash
-set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
-
-VERSION="$1"
-IMPORT_DIR="$2"
-TARGET_DIR="$3"
-APPS="$4"
-
-while read -r APP; do
-  APP_PATH="$TARGET_DIR/$APP";
-  git init --bare "$APP_PATH" > /dev/null
-  PRERECEIVE_HOOK="$APP_PATH/hooks/pre-receive"
-  cat > "$PRERECEIVE_HOOK" <<EOF
-#!/usr/bin/env bash
-set -e; set -o pipefail;
-
-cat | DOKKU_ROOT="$DOKKU_ROOT" dokku git-hook "$APP"
-EOF
-  chmod +x "$PRERECEIVE_HOOK"
-done < "$APPS" > /dev/null