Revert "rustc-1.69.0 Build 10095304"

This revert was created by Android Culprit Assistant. The culprit was identified in the following culprit search session (http://go/aca-get/1fa75415-4722-46e3-b61b-7de26844a12d).

Change-Id: I7aabd3ff76c255ca13d594cc0f2f331ed8edc01b
diff --git a/rust/config/global.go b/rust/config/global.go
index 748bb3d..2d1f0c1 100644
--- a/rust/config/global.go
+++ b/rust/config/global.go
@@ -24,7 +24,7 @@
 var pctx = android.NewPackageContext("android/soong/rust/config")
 
 var (
-	RustDefaultVersion = "1.69.0"
+	RustDefaultVersion = "1.68.0"
 	RustDefaultBase    = "prebuilts/rust/"
 	DefaultEdition     = "2021"
 	Stdlibs            = []string{