[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>
1 file changed