[SPARK-12734][HOTFIX][TEST-MAVEN] Fix bug in Netty exclusions

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 https://github.com/apache/spark/compare/3119206b7188c23055621dfeaf6874f21c711a82...5211ab8#diff-600376dffeb79835ede4a0b285078036

Author: Josh Rosen <joshrosen@databricks.com>

Closes #10693 from JoshRosen/netty-hotfix.