[roll third_party/pigweed] pw_rpc: Remove accidental Python dependency

pw_build is only needed for tests.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/39988

third_party/pigweed Rolled-Commits: 9b085ce0cd03b4c..2981da2fbef7b80
Roller-URL: https://ci.chromium.org/b/8850678913318989888
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8850678913318989888
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I176631c122d2ee8e35506fd16a5471ccdf9655f6
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/40042
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 9b085ce..2981da2 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 9b085ce0cd03b4cf663b2ba11cc5e1f7e1f82f6f
+Subproject commit 2981da2fbef7b80366a0c7d7931b651cb0c9198c