Remove libraryDependency exclude from build

[ashawley]

Is this needed any longer?

https://github.com/scala/scala-xml/blob/5dac609d66/build.sbt#L137

Is the scala-compiler dependency an actual dependency? If the point is to avoid getting a redundant scala-xml on the classpath in this build or downstream users, that shouldn't be a problem with sbt 1.2 or later. I know there are compiler and reflection tests, but maybe they can be moved elsewhere or sunset.

If the point is to avoid a redundant scala-xml dependency from the compiler for downstream users, that was fixed in 2.13. It seems it shouldn't be an issue with 2.12, but it would take an investigation.