jsinterop-annotations has its own repo on github. This CL removes
the code from the gwt repo and add a dependency to the jar file
now present in tools repo. The gwt-user.jar distributed in the zip
will still contain these classes, but the maven version will correctly
reference the jar already present in maven central.

Change-Id: I5c341e1ae5ad48ea3bf8104d781b1d16fccdef4a