Stop globally disabling unused-imports lint when compiling Rust.

https://github.com/rust-lang/rust/issues/121708#issuecomment-2075956987
points out that new linting scenarios have been moved into a separate
lint.  Thanks to this, we no longer need to globally disable this lint
in Chromium (some narrower exclusions still need to apply).

Bug: chromium:326247202
Change-Id: I8b919d0d04eb057e351d6a21bdb059902d0fe753
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6130365
Reviewed-by: Nabil Wadih <nwadih@google.com>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1401585}

build/config/compiler/BUILD.gn

@@ -1823,9 +1823,6 @@ config("treat_warnings_as_errors") {
   # Note we apply the actual lint flags in config("compiler"). All warnings
   # are suppressed in third-party crates.
   rustflags = [ "-Dwarnings" ]
-
-  # TODO(https://crbug.com/326247202): Fix unused imports and remove this flag.
-  rustflags += [ "-Aunused-imports" ]
 }
 
 # default_warnings ------------------------------------------------------------

third_party/beto-core/BUILD.gn

@@ -392,6 +392,9 @@ rust_static_library("test_helper") {
     "//third_party/rust/itertools/v0_11:lib",
     "//third_party/rust/serde_json/v1:lib",
   ]
+
+  # TODO(https://crbug.com/326247202): Fix unused imports and remove this flag.
+  rustflags = [ "-Aunused-imports" ]
 }
 
 rust_static_library("rand_ext") {