fix: remove outdated Updater support

[cmer81]

updatefile.txt was last touched in 2023: it still advertised 1.7.4 (plugin is 1.7.15), listed a source file that no longer exists (lilac_anti_duck_delay.sp) and missed lilac_database.sp, 7 translations and the includes.

Updating it wouldn't have been enough: Updater downloads each file from the update file's directory with Path_SM/ stripped, so with the current repo layout plugins/lilac.smx and translations/* would 404 (the .smx is only shipped in the release archive and translations live under addons/sourcemod/). Bumping Latest would only have made servers with lilac_auto_update 1 attempt a broken update.

Nobody in the org uses lilac_auto_update (default 0) and deployment goes through the CI releases, so this removes Updater support entirely:

• delete updatefile.txt
• remove UPDATE_URL, the Updater natives, lilac_update_url() and the lilac_auto_update ConVar (CVAR indexes renumbered so the CVAR_MAX loop stays valid)
• drop Updater from the README optional deps, add a Changelog entry
• bump version to 1.7.16

A leftover lilac_auto_update line in an existing config is simply ignored.

Compiled with spcomp 1.13.0.7427: no errors, no new warnings vs master.

Closes #49