perf: remove dead relayTimes field (#1931)

The `relayTimes` field (`map[string][]int64`) on `PacketStore` is never
written to and never read. Its only references are the declaration at
`store.go:180` and the `make()` in `NewPacketStore` at `store.go:644`.

`relay_liveness_test.go` looks like a user at a glance but builds its
own local `idx := make(map[string][]int64)` and passes that to
`addTxToRelayTimeIndex`; the string "relayTimes" there is only inside a
`t.Error` message.

This is the surviving fragment of #1872, which no longer compiles after
#1855 removed `lastSeenTouched` and `touchRelayLastSeen` from master.
Verified against current master: both references are gone, build passes,
all tests pass (32.2s).

Co-authored-by: Joel Claw <358739783+Joel-Claw@users.noreply.github.com>

cmd/server/store.go

@@ -177,7 +177,6 @@ type PacketStore struct {
 	byNode        map[string][]*StoreTx      // pubkey → transmissions
 	nodeHashes    map[string]map[string]bool // pubkey → Set<hash>
 	byPathHop     map[string][]*StoreTx      // lowercase hop/pubkey → transmissions with that hop in path
-	relayTimes    map[string][]int64         // lowercase pubkey → sorted unix-millis of relay events (full pubkeys only)
 	byPayloadType map[int][]*StoreTx         // payload_type → transmissions
 	loaded        bool
 	totalObs      int
@@ -641,7 +640,6 @@ func NewPacketStore(db *DB, cfg *PacketStoreConfig, cacheTTLs ...map[string]inte
 		byObserver:    make(map[string][]*StoreObs),
 		byNode:        make(map[string][]*StoreTx),
 		byPathHop:     make(map[string][]*StoreTx),
-		relayTimes:    make(map[string][]int64),
 		nodeHashes:    make(map[string]map[string]bool),
 		byPayloadType: make(map[int][]*StoreTx),
 		rfCache:       make(map[string]*cachedResult),