Remove compatibility stuff from the core

Stuff that still needs to be looked at:

- `CreateMultiForwardEx` (CForward.cpp) needs to be reviewed, since it's completely useless without compatability layer
- `CPluginMngr::ReadIntoOrFromCache` relies on `IsOldFile()`
- `load_amxscript` (modules.cpp) doesn't fail on `CAmxxReader::Err_OldFile`
- `C_Spawn` (meta_api.cpp) still sets `amx_` localinfos
- `insert_path` (sclist.c) adds `amxmodx_compat` as an extra path