Remove Block Broadcast Fallback config

Companion to KamadoPool's df5e65d. The fallback submitter has been
scrapped — primary bitcoind remains the sole submission target, with
upstream's indefinite-retry behavior preserved. Drops the
block-broadcast-fallback union from advanced config and the
BACKUP_RPC_URLS / PENDING_BLOCKS_DIR / PENDING_BLOCKS_GRACE exports
from docker_entrypoint.

docker_entrypoint.sh

@@ -63,30 +63,6 @@ else
 fi
 export MEMPOOL_BASE_URL
 
-# Block-broadcast fallback: if the user enabled the safety-net
-# broadcaster AND provided at least one backup RPC URL, point
-# kamado-api at the patched-ckpool sidecar dir and feed the URL list
-# through. Empty PENDING_BLOCKS_DIR disables the watcher entirely.
-FALLBACK_ENABLED=$(q '.advanced.block-broadcast-fallback.enabled // false')
-if [[ "${FALLBACK_ENABLED}" == "true" ]]; then
-    # Newline-joined list — matches kamado-api's parser. q strips
-    # quotes via -r, and yq emits one entry per line for [] expansion.
-    BACKUP_RPC_URLS=$(q '.advanced.block-broadcast-fallback.backup-rpcs[]' 2>/dev/null || true)
-    if [[ -z "${BACKUP_RPC_URLS}" ]]; then
-        echo "Block-broadcast fallback enabled but no backup RPC URLs listed — disabling." >&2
-        export PENDING_BLOCKS_DIR=""
-        export BACKUP_RPC_URLS=""
-    else
-        # ckpool's patch writes hex files to <logdir>/pending-blocks.
-        export PENDING_BLOCKS_DIR="${LOGDIR}/pending-blocks"
-        export BACKUP_RPC_URLS
-        export PENDING_BLOCKS_GRACE=$(q '.advanced.block-broadcast-fallback.grace-seconds // 30')s
-    fi
-else
-    export PENDING_BLOCKS_DIR=""
-    export BACKUP_RPC_URLS=""
-fi
-
 # CKPool-solo uses the worker's stratum username as the payout
 # address and refuses to authenticate workers whose username is not
 # a valid address on the active network. The conf `btcaddress` is