Update preferences.h

* fix const correctness of `prefs_load()`
* remove some API's that don't exist anymore

Signed-off-by: Steffen Jaeckel <jaeckel-floss@eyet-services.de>

src/config/preferences.c

@@ -251,7 +251,7 @@ prefs_reload(void)
 }
 
 void
-prefs_load(char* config_file)
+prefs_load(const char* config_file)
 {
     if (config_file == NULL) {
         prefs_loc = files_get_config_path(FILE_PROFRC);