[roll third_party/pigweed] pw_stream: Remove deprecated NullWriter alias

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/57742

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: afb861c1eca3fe2..a0d6ab09f4f189a
Roller-URL: https://ci.chromium.org/b/8838432677399405360
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8838432677399405360
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ic363c166d54005303f11eeab44a5d2581be9ed5c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/57746
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index afb861c..a0d6ab0 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit afb861c1eca3fe2a1f1f4e3d35ff1086d748c44c
+Subproject commit a0d6ab09f4f189aae97a83414f03a2fb7bcf8a2b