Multitopicproducer is not official spec for pulsar-client

[DonghunLouisLee]

I believe multi topic producer is not a required spec for pulsar clients and I think it's highly unnecessary with misleading configurations such as "batch" being separate configs for each producers.

I'm thinking of removing this feature since maintaining this sorts of does not align with implementing #213.

[DonghunLouisLee]

moreover, pulsar client struct does not need to have the ability to produce messages directly. Producer should be built to send messages only. Having client struct impl send, send_raw is unnecessary.

[tisonkun]

Why do we implement this struct in the first place?