replace tempdir by tempfile (#2143)

## Issue Addressed

Fixes #2141 
Remove [tempdir](https://docs.rs/tempdir/0.3.7/tempdir/) in favor of [tempfile](https://docs.rs/tempfile/3.1.0/tempfile/).

## Proposed Changes

`tempfile` has a slightly different api that makes creating temp folders with a name prefix a chore (`tempdir::TempDir::new("toto")` => `tempfile::Builder::new().prefix("toto").tempdir()`).

So I removed temp folder name prefix where I deemed it not useful.

Otherwise, the functionality is the same.

testing/remote_signer_test/src/utils.rs

@@ -11,7 +11,7 @@ use std::io::Write;
 use std::net::IpAddr::{V4, V6};
 use std::os::unix::fs::PermissionsExt;
 use std::path::Path;
-use tempdir::TempDir;
+use tempfile::TempDir;
 use types::{
     AggregateSignature, Attestation, AttestationData, AttesterSlashing, BeaconBlock,
     BeaconBlockHeader, BitList, Checkpoint, Deposit, DepositData, Epoch, EthSpec, FixedVector,