Remove support for "default [Map] key serializer" configuration from `SerializerProvider`

[cowtowncoder]

One goal of 3.0 is to simplify APIs, including configuration; as well as to change key entities immutable where possible. One of legacy one-off features has been ability to specify a single fallback serializer to be used for Map key types for which no key serializer can be located using other facilities, such as pluggable modules.

But with combination of much extended standard key serializers and flexibility of Modules, this feature seems obsolete by now. Worse, it is part of mutable configuration of SerializerProvider, beyond basic blueprint/instance setup that we want to support (where SerializerProvider [which ought to be named "SerializationContext", is per-call state; separate from stable SerializationConfig`).

So: let's eliminate this first. Then we can see what to do with custom null key / null serializer handlers.