fix: silence unused withLock result warning in OutlineImageProvider

lock.withLock { inFlight.remove(url) } inside the defer returned
Set.remove's String? result unused - withLock mirrors its closure's
return type, so an unused Set.remove leaked through as an unused
withLock result. Explicitly discard it inside the closure instead.

Outpost/Support/OutlineImageProvider.swift

@@ -61,7 +61,7 @@ final class OutlineImageProvider: EmbeddedImageProvider, @unchecked Sendable {
         guard !alreadyLoading else { return }
 
         Task {
-            defer { lock.withLock { inFlight.remove(url) } }
+            defer { lock.withLock { _ = inFlight.remove(url) } }
             do {
                 let data: Data
                 if url.hasPrefix("http://") || url.hasPrefix("https://"), let externalURL = URL(string: url) {