Remove override models for next Garry's Mod update

[ZehMatt]

Garry's Mod provides them now out of the box, might need to keep some.

https://commits.facepunch.com/481021 https://commits.facepunch.com/482428

[knoxed]

The checked NPCs have their models combined by the game. NPCs written in bold are tricky to combine.

• [x] Alyx
• [ ] Advisor
• [ ] Antlion
• [ ] Combine Dropship
• [ ] Dog
• [ ] Eli
• [ ] Fast zombie
• [x] Barney
• [x] Blackout (EP1 Intro)
• [x] Kleiner
• [ ] GMan
• [ ] Lamarr
• [x] Vortigaunt
• [ ] Mossman
• [ ] Zombie