core: Avoid Set.removeAll() when passing a possibly-large List (grpc#11994)

See grpc#11958

Author: ejona86

core/src/main/java/io/grpc/internal/DelayedClientTransport.java

@@ -321,7 +321,11 @@ final void reprocess(@Nullable SubchannelPicker picker) {
       if (!hasPendingStreams()) {
         return;
       }
-      pendingStreams.removeAll(toRemove);
+      // Avoid pendingStreams.removeAll() as it can degrade to calling toRemove.contains() for each
+      // element in pendingStreams.
+      for (PendingStream stream : toRemove) {
+        pendingStreams.remove(stream);
+      }
       // Because delayed transport is long-lived, we take this opportunity to down-size the
       // hashmap.
       if (pendingStreams.isEmpty()) {