[roll third_party/pigweed] README.md: Remove some unnecessary material

- The docs are now available online.
- Remove reference to a module that probably shouldn't be used.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/34040

third_party/pigweed Rolled-Commits: 38f8708718ceee9..ce7ef09c8a6e972
CQ-Do-Not-Cancel-Tryjobs: true
Roller-URL: https://ci.chromium.org/b/8854208914689339152
Cq-Cl-Tag: roller-builder:pigweed-sample-project-roller
Cq-Cl-Tag: roller-bid:8854208914689339152
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I288292b17d528c335b93f97930afed912f960375
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sample_project/+/34440
Reviewed-by: 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 38f8708..ce7ef09 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 38f8708718ceee921e7336df01d722b20743eb6d
+Subproject commit ce7ef09c8a6e972152c4c6f4cbf345c502b2353f