Remove support for Netty4ClientHttpRequestFactory in RestTemplateBuilder

[bclozel]

I'm in favor of removing the automatic configuration of that 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+.