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