[HL2MP] Fix hud_showtargetid doing nothing

[speedvoltage]

Issue: The client console ConVar hud_showtargetid does... absolutely nothing. It is declared but never referenced elsewhere in the code, meaning that setting it to 0 has no effect on hiding the target ID when aiming at a player.

Fix: Implement the missing functionality by checking the value of hud_showtargetid and conditionally displaying or hiding the HUD target ID when the player aims at another player.