Remove Guava from poms

Prevents Guava from leaking into an application using dagger-compiler.

This isn't a normal case of a leaking dependency -- it doesn't make it into final jars because (per the README) dagger-complier is marked provided. However, it makes it into IDEs, of course, and since non-existent annotations are allowed on a class, transitive annotations (such as Guava's @Subscribe) can be a problem.