refactor(key-wallet): drop WalletInfoInterface::set_birth_height

The setter only updated `birth_height`, leaving `synced_height` and `last_processed_height` at their previous values, so a caller using it after construction would silently bypass the seeding invariant established by `from_wallet`. With the per-wallet height tracking in #694 that aggregate to a min/max across wallets, this could drag the manager's `synced_height` back to genesis. After the constructor change, the setter has zero callers in-tree, so removing it is the cleanest way to enforce the invariant: `birth_height` is fixed at construction via `from_wallet`.

Addresses CodeRabbit review comment on PR #692
#692 (comment)

Author: xdustinface

key-wallet/src/wallet/managed_wallet_info/wallet_info_interface.rs

@@ -52,9 +52,6 @@ pub trait WalletInfoInterface: Sized + WalletTransactionChecker + ManagedAccount
     /// Get the birth height of the wallet
     fn birth_height(&self) -> CoreBlockHeight;
 
-    /// Set the birth height
-    fn set_birth_height(&mut self, height: CoreBlockHeight);
-
     /// Get the timestamp when first loaded
     fn first_loaded_at(&self) -> u64;
 
@@ -166,10 +163,6 @@ impl WalletInfoInterface for ManagedWalletInfo {
         self.metadata.birth_height
     }
 
-    fn set_birth_height(&mut self, height: CoreBlockHeight) {
-        self.metadata.birth_height = height;
-    }
-
     fn last_processed_height(&self) -> CoreBlockHeight {
         self.metadata.last_processed_height
     }