Remove unused -C link-arg=-fuse-ld=lld (#15097)

That's a gcc flag. lld is a linker, not a C compiler, and doesn't accept
a `use-ld` flag. `-C linker=rust-lld` (which is already present in the
test) is enough.

This fixes the following warning found in rust-lang/rust#119286:
```
---- expected: tests\testsuite\freshness.rs:2822:27
++++ actual:   stderr
   1    1 | [FRESH] foo v0.1.0 ([ROOT]/foo)
        2 + [WARNING] linker stderr: rust-lld: ignoring unknown argument '-fuse-ld=lld'␍
        3 +   |
        4 +   = [NOTE] `#[warn(linker_messages)]` on by default
        5 +
        6 + [WARNING] `foo` (lib) generated 1 warning
   2    7 | [FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
```

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tests/testsuite/freshness.rs

@@ -2799,7 +2799,6 @@ fn lld_is_fresh() {
             r#"
                 [target.x86_64-pc-windows-msvc]
                 linker = "rust-lld"
-                rustflags = ["-C", "link-arg=-fuse-ld=lld"]
             "#,
         )
         .file(

tests/testsuite/freshness_checksum.rs

@@ -2602,7 +2602,6 @@ fn lld_is_fresh() {
             r#"
                 [target.x86_64-pc-windows-msvc]
                 linker = "rust-lld"
-                rustflags = ["-C", "link-arg=-fuse-ld=lld"]
             "#,
         )
         .file(