[rust] No need to explicitly opt-into -Dunsafe_op_in_unsafe_fn.

rust-lang/rust#71668 points out that
rust-lang/rust#112038 made the
`unsafe_op_in_unsafe_fn` lint `warn`-by-default from edition 2024
onward.  (https://crbug.com/450891119 tracks history of when Chromium
switched to using edition 2024 as the default.)

Bug: 450891119
Change-Id: I1ed687c8a975e5a35c7ab38291c838bb13ff8c19
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7361485
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Auto-Submit: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1564439}
NOKEYCHECK=True
GitOrigin-RevId: fc831996c3cee841cead31299a7c36f97f51a225

Author: anforowicz

config/compiler/BUILD.gn

@@ -2273,13 +2273,6 @@ config("default_warnings") {
   if (is_apple) {
     ldflags += [ "-Wl,-no_warn_duplicate_libraries" ]
   }
-
-  # Rust warnings
-
-  # Require `unsafe` blocks even in `unsafe` fns. This is intended to become
-  # an error by default eventually; see
-  # https://github.com/rust-lang/rust/issues/71668
-  rustflags = [ "-Dunsafe_op_in_unsafe_fn" ]
 }
 
 # prevent_unsafe_narrowing ----------------------------------------------------