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/Cargo.toml

@@ -29,7 +29,3 @@ clippy = { version = "^0.*", optional = true }
 [[test]]
 name = "test"
 path = "tests/test.rs"
-
-[[bench]]
-name = "bench"
-path = "benches/bench.rs"