fix(BA-6085): make migration downgrade a no-op

The prior revision 7ea9f3c1b2d5 already sets custom.reads_vfolder_config_files
to TRUE during column creation, so unconditionally writing FALSE on downgrade
clobbered that valid prior state. Drop the UPDATE so downgrade leaves the row
untouched.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: seedspirit

src/ai/backend/manager/models/alembic/versions/0b10b2c6a972_seed_reads_vfolder_for_custom_variant.py

@@ -36,8 +36,4 @@ def upgrade() -> None:
 
 
 def downgrade() -> None:
-    op.execute(
-        sa.text(
-            "UPDATE runtime_variants SET reads_vfolder_config_files = false WHERE name = 'custom'"
-        )
-    )
+    pass