fix(testnet): remove empty metadata env override causing frontend crash

Empty NEXT_PUBLIC_METADATA_SERVICE_API_HOST= in testnet override
conflicts with NEXT_PUBLIC_METADATA_ADDRESS_TAGS_UPDATE_ENABLED.
Let base (mainnet) metadata config apply — Blockscout metadata
service is public and works for testnet too.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: JOY

docker-compose/envs/common-frontend-testnet.env

@@ -36,9 +36,5 @@ NEXT_PUBLIC_USER_OPS_INDEXER_API_HOST=https://test-ops.doscan.io
 NEXT_PUBLIC_STATS_API_HOST=https://test-stats.doscan.io
 NEXT_PUBLIC_VISUALIZE_API_HOST=https://test-viz.doscan.io
 
-# Metadata - disable on testnet (no service available)
-NEXT_PUBLIC_METADATA_SERVICE_API_HOST=
-NEXT_PUBLIC_METADATA_ADDRESS_TAGS_UPDATE_ENABLED=false
-
 # Metasuites - disable on testnet
 NEXT_PUBLIC_METASUITES_ENABLED=false