Remove Guava from dagger

Seems to have been unused, and was "leaking" into applications via
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.