[gn][rust] Cleanup soft-migration source enforcement opt-in

Change-Id: Id17abb9b645536d692be8a7352dea4daa3c4c70d
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/boringssl/+/414774
Reviewed-by: Yu Shan <shanyu@google.com>
Commit-Queue: Adam Lesinski <adamlesinski@google.com>
diff --git a/rust/boringssl-sys/BUILD.gn b/rust/boringssl-sys/BUILD.gn
index 6a01da3..3339215 100644
--- a/rust/boringssl-sys/BUILD.gn
+++ b/rust/boringssl-sys/BUILD.gn
@@ -15,8 +15,6 @@
     "//third_party/boringssl:crypto",
   ]
 
-  enforce_source_listing = true
-
   sources = [ "src/lib.rs" ]
 
   if (current_toolchain == host_toolchain) {