[HOTFIX][test-maven][SPARK-12734] Fix bug in Netty exclusions

[JoshRosen]

This is a hotfix for a build bug introduced by the Netty exclusion changes in #10672. We can't exclude io.netty:netty because Akka depends on it. There's not a direct conflict between io.netty:netty and io.netty:netty-all, because the former puts classes in the org.jboss.netty namespace while the latter uses the io.netty namespace. However, there still is a conflict between org.jboss.netty:netty and io.netty:netty, so we need to continue to exclude the JBoss version of that artifact.

While the diff here looks somewhat large, note that this is only a revert of a some of the changes from #10672. You can see the net changes in pom.xml at 3119206...5211ab8#diff-600376dffeb79835ede4a0b285078036

[SparkQA]

Test build #49104 has finished for PR 10693 at commit 1d24aaa.

• This patch fails Scala style tests.
• This patch merges cleanly.
• This patch adds no public classes.