[HL2MP] Death fall black screen removal

[speedvoltage]

Issue: When a player die by fall damage, the screen becomes black.

Fix: Remove the black fade, because it is unnecessary.

[Bitl]

Why not move

virtual bool  FlPlayerFallDeathDoesScreenFade( CBasePlayer *pl ) { return false; }

to hl2mp_gamerules.h rather than do it in the base gamerules class? The removal of code in movehelper_server.cpp is probably not needed either as it already checks if FlPlayerFallDeathDoesScreenFade is enabled. (the PR also removes the "return false" on death in movehelper_server.cpp, which isn't ideal as that function should call "return false" if the player is dead)