roll: third_party/pigweed docs: Remove unused imports

CL 348973 made the `json` package unneeded.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/349252
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Original-Revision: 0537186bf0d666e4aed2732e6ac09aa1f6c088a6

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: 0d38b288172b26..0537186bf0d666
Roll-Count: 1
Roller-URL: https://cr-buildbucket.appspot.com/build/8697127556497613041
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I5ef13a75396f50233e77d75942b83126ca334c56
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/examples/+/349312
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
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diff --git a/third_party/pigweed b/third_party/pigweed
index 0d38b28..0537186 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 0d38b288172b26d0324abfe4fc525466c2d6dd1b
+Subproject commit 0537186bf0d666e4aed2732e6ac09aa1f6c088a6