chore(tags): stop auto-generating type:/editor:/favorited_by: tags

villebro noted on #43390 that these system-generated tags appear to be
unused. Confirmed: every tags list and filter in the frontend explicitly
excludes non-custom tags (ChartList, DashboardList, SavedQueryList, the
chart PropertiesModal, the dashboard Header), so nothing ever surfaced
them to a user. What remained was pure write-side overhead: 13
SQLAlchemy event listeners across 5 models firing on every chart/
dashboard/query/dataset save and every favorite/unfavorite, plus a whole
performance-optimization mixin (CustomTagsOptimizationMixin,
DASHBOARD_LIST_CUSTOM_TAGS_ONLY) that existed purely to strip the
resulting noise back out of dashboard-list responses.

This removes the generation:
- superset/tags/models.py: drop ObjectUpdater's editor:/type: generation
  (after_insert/after_update) and FavStarUpdater's favorited_by:
  generation entirely. Keeps after_delete (tagged_object cleanup applies
  to every tag, custom included, and nothing else removes those rows
  since tagged_object.object_id has no FK - see its column comment).
- superset/tags/core.py: only registers the delete-cleanup listeners now.
- superset/common/tags.py + the `sync_tags` CLI command: removed (the
  backfill path for the generation this removes).
- superset/views/custom_tags_api_mixin.py, DASHBOARD_LIST_CUSTOM_TAGS_ONLY,
  Dashboard.custom_tags, and the schema/API plumbing built around them:
  removed - nothing left to optimize away once implicit tags stop
  accumulating.

Kept for backward compatibility, since MCP's list_tags/get_tag_info tools
document these tag types and upgraded deployments may already have rows
of these types: the TagType enum values, the custom_tag API filter,
and bulk-delete protection for non-custom tags. Docstrings updated to
say these are legacy/no longer generated rather than actively implicit.

Also fixes a real, currently-broken import in superset/daos/tag.py
(current_user_can_modify_object doesn't live in
superset.commands.tag.utils, only in superset.commands.utils) that
otherwise blocks every test in this area from running at all. Filed and
fixed separately as #43467; this commit will collapse away on rebase
once that merges.

Follow-up to #43390.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/static/resources/openapi.json

@@ -3257,12 +3257,6 @@
             "description": "Override CSS for the dashboard.",
             "type": "string"
           },
-          "custom_tags": {
-            "items": {
-              "$ref": "#/components/schemas/Tag1"
-            },
-            "type": "array"
-          },
           "dashboard_title": {
             "description": "A title for the dashboard.",
             "type": "string"
@@ -17207,7 +17201,6 @@
                     "charts": [],
                     "created_on_delta_humanized": "string",
                     "css": "string",
-                    "custom_tags": [],
                     "dashboard_title": "string",
                     "id": 1,
                     "is_managed_externally": true,