refactor(net): remove the dead java config module

[BryanFRD]

Problem

net/java/config/ is a closed island of 337 lines that nothing reaches. The Java
config phase was reimplemented on PendingConnection in net/java/pending.rs and
net/java/login/, and the old impl JavaClient version was never deleted. The
matching names on the two types make it easy to miss.

Every handler has a live replacement:

Removed (impl JavaClient)	Live (impl PendingConnection)
handle_client_information_config	pending.rs
handle_plugin_message	pending.rs
handle_resource_pack_response	pending.rs
handle_config_cookie_response	pending.rs
handle_known_packs	login/known_packs.rs
send_known_packs	login/login_acknowledged.rs
handle_config_acknowledged	superseded by Registry::get_synced(version)
handle_config_keep_alive	obsolete: keep-alives are only sent in the play loop, the pending phase uses HANDSHAKE_IDLE_TIMEOUT

handle_config_acknowledged and send_known_packs were only called from other
functions inside the module. build_dimension_nbt was re-exported to the parent
with no consumer, and BRAND_CHANNEL_PREFIX is duplicated verbatim in pending.rs.

Change

Delete the directory, its pub mod config; declaration, and the
PacketHandlerResult import it was the last user of.

No behaviour change: cargo check --all-features is clean, with no new warnings.