Consolidate systemd environment configuration by moving EnvironmentFile directives from drop-in files to main service files

- Move EnvironmentFile=/etc/aitbc/.env from 10-central-env.conf drop-in files directly into main service files for blockchain-node, blockchain-p2p, blockchain-rpc, blockchain-sync, coordinator-api, exchange-api, explorer, learning, marketplace, modality-optimization, multimodal, openclaw, and wallet services
- Delete all 10-central-env.conf drop-in configuration files
- Delete

systemd/aitbc-blockchain-p2p.service

@@ -10,6 +10,7 @@ WorkingDirectory=/opt/aitbc/apps/blockchain-node
 Environment=PATH=/usr/bin:/usr/local/bin:/usr/bin:/bin
 Environment=PYTHONPATH=/opt/aitbc/apps/blockchain-node/src:/opt/aitbc/apps/blockchain-node/scripts
 EnvironmentFile=/etc/aitbc/blockchain.env
+EnvironmentFile=/etc/aitbc/.env
 ExecStart=/opt/aitbc/venv/bin/python -m aitbc_chain.p2p_network --host ${p2p_bind_host} --port ${p2p_bind_port} --peers ${p2p_peers} --node-id ${p2p_node_id}
 Restart=always
 RestartSec=5