Remove: the concept of UnknownGRFs

These were filled with "<Unknown>" (before 8a2da49) and later their name would get filled via UDP requests to the server. These UDP packets do not exist anymore, so they will always remain "<Unknown>".
Remove that logic and just use the generic translated error GRF UNKNOWN string instead.

src/newgrf_config.h

@@ -234,10 +234,6 @@ char *GRFBuildParamList(char *dst, const GRFConfig *c, const char *last);
 /* In newgrf_gui.cpp */
 void ShowNewGRFSettings(bool editable, bool show_params, bool exec_changes, GRFConfig **config);
 
-/** For communication about GRFs over the network */
-#define UNKNOWN_GRF_NAME_PLACEHOLDER "<Unknown>"
-GRFTextWrapper FindUnknownGRFName(uint32 grfid, uint8 *md5sum, bool create);
-
 void UpdateNewGRFScanStatus(uint num, const char *name);
 void UpdateNewGRFConfigPalette(int32 new_value = 0);