verification: fix clippy::needless_borrows_for_generic_args

```
error: the borrowed expression implements the required traits
   --> rustls-platform-verifier/src/verification/android.rs:193:53
    |
193 |                         JValue::from(env.new_string(&server_name.to_str())?),
    |                                                     ^^^^^^^^^^^^^^^^^^^^^ help: change this to: `server_name.to_str()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrows_for_generic_args
    = note: `-D clippy::needless-borrows-for-generic-args` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::needless_borrows_for_generic_args)]`
```

rustls-platform-verifier/src/verification/android.rs

@@ -190,7 +190,7 @@ impl Verifier {
                     VERIFIER_CALL,
                     &[
                         JValue::from(*cx.application_context()),
-                        JValue::from(env.new_string(&server_name.to_str())?),
+                        JValue::from(env.new_string(server_name.to_str())?),
                         JValue::from(env.new_string(AUTH_TYPE)?),
                         JValue::from(JObject::from(allowed_ekus)),
                         JValue::from(ocsp_response),