fix(providers): drop orphaned Secret-model migrations

0108_alter_secret_key_version_to_char and 0110_merge_0108_branches
came in via the shipping-platform sync, but reference a Secret model
that never existed upstream. The 10 source files (models/secret.py,
credential_manager.py, secret_manager.py, rotation.py, management
commands, the 0102 initial secret-storage migration, etc.) were
phantom-deletions in the sync patch and intentionally skipped — so
the 'alter' migration has nothing to alter and the 'merge' migration
only existed to reconcile that dead branch.

Chain is now linear again: 0107 -> 0108_clear_dhl_parcel_de... ->
0109_cleanup_legacy_system_rate_sheets.

Author: danh91