This commit re-enables thirdPartyAudit for x-pack core. Previously, when
xpack was a single plugin, it transitively picked up httpcore-nio
through the elasticsearch rest client. Now that xpack core does not
depend on the rest client, httpcore-nio must be added as a dependency.
Additionally, commons-logging was previously handled through the rest
client, but now xpack depends directly on this, thus excludes must be
added for the pesky missing classes there.
This commit also cleans up unnecessary parts of plugin/build.gradle no
longer necessary.
Original commit: elastic/x-pack-elasticsearch@70e936bdc3
9d87b63ca4