client/context: remove 'quit' method
client/context: remove 'quit' method

Remove 'Context::quit' due to its ambiguity. The quit method originally
made sense when sharding worked differently internally, but now it has
an odd meaning.

It can be read as either quitting (shutting down) the client, shutting
down the shard without restart, or shutting down the shard with restart.
All of these can be accomplished in different ways depending on what you
want to do, mainly through the client's 'ShardManager':

<https://github.com/serenity-rs/serenity/blob/v0.6.x/src/client/bridge/gateway/shard_manager.rs#L258-L313>

The three use cases each have methods on the ShardManager:

1. shutting down the client via 'ShardManager::shutdown_all'
2. shutting down a shard via 'ShardManager::shutdown(ShardId)'
3. restarting a shard via 'ShardManager::restart(ShardId)'

As a side-effect, this closes #284. v0.5.x should include a deprecation
notice like, "Use the client's ShardManager instead".

Signed-off-by: Zeyla Hellyer <60d293b220186fb54f789791fc1abae9ee9d7d8f@hellyer.dev>

acdenisSK/serenity,Flat/serenity