Remove unused --nc flag and nc crate (#55962)

The `nc` crate and `zed --nc <socket>` flag were added in #34577 to let
the Claude Code integration spawn the running zed binary as a
netcat-style bridge between stdio and a Unix socket for its MCP server.

That integration was removed in #37120 in favor of the external
`@agentclientprotocol/claude-agent-acp` npm package, which dropped the
only caller of `--nc`. The flag, its dispatch in `main.rs`, and the `nc`
crate itself were left behind and have been unused since.

Nothing in the Zed codebase spawns `zed --nc` anymore, so remove the
flag and delete the crate. The unrelated `--askpass` netcat bridge (in
the `askpass` crate) is unaffected.

Release Notes:

- N/A

.github/CODEOWNERS.hold

@@ -181,7 +181,6 @@
 /crates/fs_benchmarks/ @zed-industries/infrastructure-team
 /crates/http_client/ @zed-industries/infrastructure-team
 /crates/http_client_tls/ @zed-industries/infrastructure-team
-/crates/nc/ @zed-industries/infrastructure-team
 /crates/net/ @zed-industries/infrastructure-team
 /crates/paths/ @zed-industries/infrastructure-team
 /crates/release_channel/ @zed-industries/infrastructure-team