Remove Netty4 auto-configuration in RestTemplate

This commit removes the automatic configuration of the Netty  request
factory in RestTemplateBuilder, for the following reasons:

* as of Spring 5, `Netty4ClientHttpRequestFactory` is now deprecated
* there are quite a few issues logged in Spring Framework (duplicate
headers, such as SPR-15446 and SPR-15476)
* by default, the `Netty4ClientHttpRequestFactory` is adding a
"Connection: close" request header to all outgoing requests, which
means that the underlying HTTP connection won't be reused between
requests (which is a performance problem)

In that case, using any other request factory is a better choice for
Spring Boot 2.0+.

Note that the `RestTemplateBuilder` still allows to provide it manually
with the request factory to use. Developers can still choose this option
and will be aware of its deprecation status.

Fixes gh-9150

spring-boot/pom.xml

@@ -321,11 +321,6 @@
 			<artifactId>mariadb-java-client</artifactId>
 			<scope>test</scope>
 		</dependency>
-		<dependency>
-			<groupId>io.netty</groupId>
-			<artifactId>netty-all</artifactId>
-			<scope>test</scope>
-		</dependency>
 		<dependency>
 			<groupId>org.postgresql</groupId>
 			<artifactId>postgresql</artifactId>

spring-boot/src/main/java/org/springframework/boot/web/client/RestTemplateBuilder.java

@@ -72,8 +72,6 @@ public class RestTemplateBuilder {
 				"org.springframework.http.client.OkHttp3ClientHttpRequestFactory");
 		candidates.put("com.squareup.okhttp.OkHttpClient",
 				"org.springframework.http.client.OkHttpClientHttpRequestFactory");
-		candidates.put("io.netty.channel.EventLoopGroup",
-				"org.springframework.http.client.Netty4ClientHttpRequestFactory");
 		REQUEST_FACTORY_CANDIDATES = Collections.unmodifiableMap(candidates);
 	}
 

spring-boot/src/test/java/org/springframework/boot/web/client/RestTemplateBuilderTests.java

@@ -31,7 +31,6 @@ import org.springframework.http.client.BufferingClientHttpRequestFactory;
 import org.springframework.http.client.ClientHttpRequestFactory;
 import org.springframework.http.client.ClientHttpRequestInterceptor;
 import org.springframework.http.client.HttpComponentsClientHttpRequestFactory;
-import org.springframework.http.client.Netty4ClientHttpRequestFactory;
 import org.springframework.http.client.OkHttp3ClientHttpRequestFactory;
 import org.springframework.http.client.SimpleClientHttpRequestFactory;
 import org.springframework.http.client.support.BasicAuthorizationInterceptor;
@@ -465,24 +464,6 @@ public class RestTemplateBuilderTests {
 				.isEqualTo(1234);
 	}
 
-	@Test
-	public void connectTimeoutCanBeConfiguredOnNetty4RequestFactory() {
-		ClientHttpRequestFactory requestFactory = this.builder
-				.requestFactory(Netty4ClientHttpRequestFactory.class)
-				.setConnectTimeout(1234).build().getRequestFactory();
-		assertThat(ReflectionTestUtils.getField(requestFactory, "connectTimeout"))
-				.isEqualTo(1234);
-	}
-
-	@Test
-	public void readTimeoutCanBeConfiguredOnNetty4RequestFactory() {
-		ClientHttpRequestFactory requestFactory = this.builder
-				.requestFactory(Netty4ClientHttpRequestFactory.class).setReadTimeout(1234)
-				.build().getRequestFactory();
-		assertThat(ReflectionTestUtils.getField(requestFactory, "readTimeout"))
-				.isEqualTo(1234);
-	}
-
 	@Test
 	public void connectTimeoutCanBeConfiguredOnOkHttp3RequestFactory() {
 		ClientHttpRequestFactory requestFactory = this.builder