Remove robust-http-client

[basil]

This tiny library by Kohsuke was not being consumed directly by any Jenkins or CloudBees plugins whose binaries I searched. It was only used internally by ProxyConfiguration#getInputStream, a method no longer called by anything relevant besides https://github.com/jenkinsci/xunit-plugin/blob/4bf589c23c09a1988cf3e641e48d3251563814ac/src/main/java/org/jenkinsci/plugins/xunit/util/DownloadableResourceUtil.java#L66 and https://github.com/jenkinsci/xunit-plugin/blob/4bf589c23c09a1988cf3e641e48d3251563814ac/src/main/java/org/jenkinsci/plugins/xunit/util/DownloadableResourceUtil.java#L66, both of which seem like they could work just fine without a 5-try retry loop.

Testing done

mvn clean verify -Dtest=hudson.ProxyConfigurationManagerGUITest,hudson.ProxyConfigurationManagerTest,hudson.ProxyConfigurationTest

Proposed changelog entries

N/A

Proposed upgrade guidelines

N/A

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@mention

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[A1exKH]

@basil LGTM.