Remove compatibility stuff from the core

Stuff that still needs to be looked at:

- `CPluginMngr::ReadIntoOrFromCache` relies on `IsOldFile()` - [link](https://github.com/alliedmodders/amxmodx/blob/master/amxmodx/CPlugin.cpp#L469-L470)
- `load_amxscript` doesn't fail on `CAmxxReader::Err_OldFile` - [link](https://github.com/alliedmodders/amxmodx/blob/master/amxmodx/modules.cpp#L222-L223)
- `C_Spawn` still sets `amx_` localinfos - [link](https://github.com/alliedmodders/amxmodx/blob/master/amxmodx/meta_api.cpp#L400-L406), probably can be left there
- `insert_path` adds `amxmodx_compat` as an extra path - [link](https://github.com/alliedmodders/amxmodx/blob/master/compiler/libpc300/sclist.c#L237-L249)

I"ve also searched, this doesn't seem to be used except amxmod_compat.