refactor: use only a single implementation for cache purging

This change moves cache clearing into the repo plugin, and standardizes on a single way of clearing the cache.

plugins/apps/pre-delete

@@ -1,21 +0,0 @@
-#!/usr/bin/env bash
-set -eo pipefail
-[[ $DOKKU_TRACE ]] && set -x
-source "$PLUGIN_CORE_AVAILABLE_PATH/common/functions"
-
-apps_pre_delete() {
-  declare desc="apps pre-delete plugin trigger"
-  local trigger="apps_pre_delete"
-  local APP="$1"
-  local IMAGE_TAG="$2"
-  local CACHE_DIR="$DOKKU_ROOT/$APP/cache"
-  local CACHE_HOST_DIR="$DOKKU_HOST_ROOT/$APP/cache"
-  verify_app_name "$APP"
-
-  if [[ -d $CACHE_DIR ]]; then
-    DOKKU_IMAGE="$(config_get "$APP" DOKKU_IMAGE || echo "$DOKKU_IMAGE")"
-    "$DOCKER_BIN" run "$DOKKU_GLOBAL_RUN_ARGS" --rm -v "$CACHE_HOST_DIR:/cache" "$DOKKU_IMAGE" find /cache -depth -mindepth 1 -maxdepth 1 -exec rm -Rf {} \; || true
-  fi
-}
-
-apps_pre_delete "$@"