CLEANUP: Remove DOMAINNAME-based local client tracking

CLEANUP: Remove DOMAINNAME-based local client tracking

The ircd tracked "local clients" by matching connected users'
sockhost against *DOMAINNAME, a relic from when IRC servers ran
at universities and tracking same-host users was meaningful.
This is no longer a relevant metric.

Removes:
- lu_lulocal variable and all DOMAINNAME mask matching in
  check_lusers(), m_lusers(), and exit_one_client()
- local_count field from current_load_struct and load_entry
- DOMAINNAME define from config.h and setup.h
- The "DOMAINNAME clients" row from userload stats output

Keeps:
- lu_lu / m_client (MyConnect clients = clients connected to this
  server) which is the standard IRC "local users" for LUSERS
- All global user counters and max tracking

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>