refactor: remove unreachable match arms for tests/benches

To justify why it won't affect any existing behavior:

* For the default set of Cargo targets,
  test/bench targets would only be selected by `cargo test`,
  whose UserIntent will turn into `CompileMode::Test`,
  but never `CompileMode::Build`.

* For selective Cargo targets, i.e., `--tests`, or `--test`,
  Their `CompileMode` would either be `Check { test }` or `Test`.
  They will never be `Build`.
  See https://github.com/rust-lang/cargo/blob/bffece899e9/src/cargo/ops/cargo_compile/unit_generator.rs#L450-L465

According to these facts, there won't be a case that
bench/test targets get assigned to `CompileMode::Build`.

Author: weihanglo

src/cargo/ops/cargo_compile/unit_generator.rs

@@ -84,11 +84,6 @@ impl<'a> UnitGenerator<'a, '_> {
                     CompileMode::Test
                 }
             }
-            CompileMode::Build => match *target.kind() {
-                TargetKind::Test => CompileMode::Test,
-                TargetKind::Bench => CompileMode::Test,
-                _ => CompileMode::Build,
-            },
             _ => initial_target_mode,
         };