Remove unused MessageValue variants

[rukai]

I have vague guesses at what these were for but they are unused so lets get rid of them.

[benbromhead]

We can always add them back in, but the idea was to represent other data types (esp documents... say for mongodb).

[github-actions]

1 benchmark regressed. 0 benchmark improved. Please check the benchmark workflow logs for full details: https://github.com/shotover/shotover-proxy/actions/runs/3012197790

�[2m2022-09-08T04:03:44.671312Z�[0m �[31mERROR�[0m �[2mshotover_proxy::server�[0m�[2m:�[0m source CassandraSource encountered an error when flushing the chain for shutdown: Failed to connect to upstream: "127.0.0.1:9043"
cassandra/tls_insert    time:   [1.2105 ms 1.2480 ms 1.2883 ms]
                        thrpt:  [776.21  elem/s 801.29  elem/s 826.09  elem/s]
                 change:
                        time:   [+24.720% +31.848% +38.957%] (p = 0.00 < 0.05)
                        thrpt:  [-28.035% -24.155% -19.820%]
                        Performance has regressed.