Fully remove GWT-RPC support externally.

Really, really closes #3680.

Possible future work:
- Be consistent about whether we put @GwtCompatible on the remaining internal CustomFieldSerializer classes. (For those files, it probably acts only as documentation (of something that is pretty obvious :)). Our open-source build used to take action when it saw `@GwtCompatible(emulated=true)`, but (a) it doesn't anymore, (b) those files don't exist externally anymore, and (c) they weren't `emulated=true` :) I believe that our internal build requires @GwtCompatible on most files in our GWT target, but I think there's an exception for GWT-target-only sources.)
- Remove some of the GwtSerializationDependencies classes. However, some of these classe are set up as *supertypes* of our prod classes, so this would require some surgery. Keeping them is harmless: It doesn't actually make the classes GWT-serializable.
- Remove the usages of serializable=true on @GwtCompatible, at least externally but maybe just entirely.

RELNOTES=[Guava types can no longer be sent over GWT-RPC.](https://groups.google.com/d/msg/guava-announce/zHZTFg7YF3o/rQNnwdHeEwAJ) Even the earlier, temporary way to reenable support (`guava.gwt.emergency_reenable_rpc`) no longer has an effect.

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