Remove benchmarks

Nobody noticed these did not compile for 3 months, so I am guessing nobody
cares. The JSON benchmarks at https://github.com/serde-rs/json-benchmark are
much more relevant.

testing/build.rs

@@ -5,13 +5,7 @@ use std::path::Path;
 
 fn main() {
     let out_dir = env::var_os("OUT_DIR").unwrap();
-
-    for &(src, dst) in &[
-        ("tests/test.rs.in", "test.rs"),
-        ("benches/bench.rs.in", "bench.rs"),
-    ] {
-        let src = Path::new(src);
-        let dst = Path::new(&out_dir).join(dst);
-        serde_codegen::expand(&src, &dst).unwrap();
-    }
+    let src = Path::new("tests/test.rs.in");
+    let dst = Path::new(&out_dir).join("test.rs");
+    serde_codegen::expand(&src, &dst).unwrap();
 }