roll: third_party/pigweed pw_async2: Remove dispatcher() from Context

Giving access to the dispatcher allows tasks to potentially post other
tasks, which my not be desirable. This API was not used in practice, so
remove it.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/388492
Original-Revision: 7cecd19c809ba85eefe79ccef2001773c6dcaa7b

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: f73851cb3f2008..7cecd19c809ba8
Roll-Count: 1
Roller-URL: https://cr-buildbucket.appspot.com/build/8686498979389948225
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Id8bc88e2d86c3ba4bc44057cbaac00be9691a813
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/examples/+/389092

third_party/pigweed

diff --git a/third_party/pigweed b/third_party/pigweed
index f73851c..7cecd19 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit f73851cb3f20083565d63c5739fa20c05d935eb9
+Subproject commit 7cecd19c809ba85eefe79ccef2001773c6dcaa7b