Remove Jetty / Netty / Apache HttpClient checks for WebClient

[mhalbritter]

With spring-projects/spring-framework#29644 the WebClient doesn't need external dependencies anymore. If no netty / jetty / Apache HttpClient is found, it falls back to the JdkClientHttpConnector which is baked into the JDK.

At least in org.springframework.boot.test.web.reactive.server.WebTestClientContextCustomizerFactory we check for the presence of netty / jetty / apache httpclient. This check can be removed, and maybe such checks are somewhere else in the codebase.

[wilkinsona]

I wonder if we should consider this a bug? Looking at the code, the checks are preventing the auto-configuration of a WebTestClient that uses the JdkClientHttpConnector.

[scottfrederick]

Closing in favor of #33640.