Remove obsolete friend UI reference from platform menu config file

[SamVanheer]

The file Half-Life/platform/steam/games/PlatformMenu.vdf is used by the main menu to load additional libraries for certain UI features.

This is only used for the server browser nowadays, but a long time ago the game had direct integration with Steam's friends UI which was integrated using this system.

The friendsui.dll library no longer exists but it's still referenced in this file. The engine ignores attempts to load this dll if the interface name matches the one used by the friends UI but changing the name allows the library to load.

There is no security issue here since the file can't be overridden by mods but it's still better to remove the reference entirely.