Cleanup keymgr.c

Cleanup keymgr.c

Three small cleanups:

1. Remove an unused keystr/dst_key_format.
2. Initialize a dst_key_state_t state with NA.
3. Update false comment about local policy (local policy only adds
   barrier on transitions to the RUMOURED state, not the UNRETENTIVE
   state).

(cherry picked from commit 189d9a2d21b44eec25245cd5189c86d7115e79af)