[roll third_party/pigweed] pw_rpc: Remove PayloadBuffer() from the public API

The PayloadBuffer() function will be removed. ChannelOutput buffers will
only be used internally by the RPC system. This removes PayloadBuffer()
from the public API and exposes it only to its current upstream users.

Original-Bug: 605
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/79800

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 6d1555ddaa5bd6d..4adef6fa36c9f92
Roller-URL: https://ci.chromium.org/b/8824476410766920625
Cq-Cl-Tag: roller-builder:pigweed-sample-project-roller
Cq-Cl-Tag: roller-bid:8824476410766920625
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I4ccf6aa9ac4c2dcb1199f04d9c4d63754871074b
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sample_project/+/79887
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 6d1555d..4adef6f 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 6d1555ddaa5bd6da49b80fbf4b46f88e82e52ebb
+Subproject commit 4adef6fa36c9f9222165e35cadb9e92e3d99477e