Remove support for URLConnectionSender

[mhalbritter]

In #42589, we make ZipkinHttpClientSender the default sender. The conditions for that bean are:

@ConditionalOnClass(HttpClient.class)
@ConditionalOnMissingBean(BytesMessageSender.class)

that means on every JDK which has HttpClient in it (which should be most of them, unless you built a custom one with jlink), ZipkinHttpClientSender will be used.

Given that, I think we can remove URLConnectionSender, which needs the zipkin-sender-urlconnection dependency on the classpath.