SpringBoot Remove remaining functionality that has been deprecated for removal in 3.4.0

I just noticed this:

https://github.com/spring-projects/spring-boot/blob/c26c7b65077caf4c5bf16699ef9fd3c73ef7cb0f/spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/client/ClientHttpRequestFactories.java#L222-L230

We should do a sweep of the code and see if there's anything else that we've missed previously.