[roll third_party/pigweed] pw_env_setup: Remove unnecessary unset calls

The _pw_cleanup variable is never set in the first place (the deleted
line was the only place it appeared in Pigweed code). The
_pw_alias_check can be made local.

Removing these spurious unsets will avoid warnings being printed when
running bootstrap from zsh.

Tested by running `source bootstrap.sh` from zsh. Without this change,
it prints the warnings; with this change, it does not.

Original-Fixed: 581
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/74182

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 50262fef24edb1f..5825e5c1b1c82d1
Roller-URL: https://ci.chromium.org/b/8827817251357229409
Cq-Cl-Tag: roller-builder:pigweed-sample-project-roller
Cq-Cl-Tag: roller-bid:8827817251357229409
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I455a73dc07b79119762183701c23af82a1cbe6e0
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sample_project/+/74381
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 50262fe..5825e5c 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 50262fef24edb1f08bae69db8787d396b5da0d9f
+Subproject commit 5825e5c1b1c82d1563cac8a992d4c5d8f81db422