[roll third_party/pigweed] third_party/emboss: Assume newer emboss version 2/2

Remove no longer used pw_third_party_emboss_USE_NEW_SOURCES GN flag
introduced in http://pwrev.dev/194876.

Note this should not be landed until downstream projects no longer
use the flag.

Original-Bug: 329872338
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/197363

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 910df6e3f5bcf8d..a4bba78b1e9a492
Roller-URL: https://ci.chromium.org/b/8752723036117947537
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I4264b9a84d106a96120c0275f45b4308d9b2b672
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/199018
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 910df6e..a4bba78 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 910df6e3f5bcf8d83c121c0a31cdaf088343b8bb
+Subproject commit a4bba78b1e9a4922345afc79feaa364135a4ebe2