Remove unused jimage compressor plugin configuration

There's an unused concept of a pluginConfig that is passed into the jlink compress plugins, however we always pass null here and the code seems broken (the pluginConfig wouldn't have been stored properly). This seem to be a left-over from a more generic plugin design that never came to fruition.

Appropriate action seems to be to clean out this code from the plugins and decompressors, while keeping the compressed header format intact for backwards compatibility (and in case we want to revisit this in the future)