[ dart:io ] Remove bad assert from socket profiling stat collection

Fixes https://github.com/dart-lang/sdk/issues/61187

CoreLibraryReviewExempt:dart-io
Change-Id: I66e06b546e9e40e58874da5d70b0f05647aac190
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/442260
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Auto-Submit: Ben Konyi <bkonyi@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>

sdk/lib/io/network_profiling.dart

@@ -271,7 +271,6 @@ abstract class _SocketProfile {
     // Skip any socket that started before `_enableSocketProfiling` was turned
     // on.
     final stats = _idToSocketStatistic[idKey];
-    assert(stats != null, '"$idKey" not found in "_idToSocketStatistic" map');
     if (stats == null) return;
     switch (type) {
       case _SocketProfileType.endTime: