Remove deprecated flag --disable-deposit-contract-sync from doc (#8124)

Co-Authored-By: 0x19dG87 <dmytro.ico@gmail.com>

Author: 0x19dG87

book/src/run_a_node.md

@@ -78,12 +78,9 @@ lighthouse bn \
   --network mainnet \
   --execution-endpoint http://localhost:8551 \
   --execution-jwt /secrets/jwt.hex \
-  --checkpoint-sync-url https://mainnet.checkpoint.sigp.io \
-  --disable-deposit-contract-sync
+  --checkpoint-sync-url https://mainnet.checkpoint.sigp.io
 ```
 
-Since we are not staking, we can use the `--disable-deposit-contract-sync` flag to disable syncing of deposit logs from the execution node.
-
 Once Lighthouse runs, we can monitor the logs to see if it is syncing correctly.
 
 ## Step 4: Check logs for sync status