Don't set preferred core for GBA when importing .vbm, there's only one

this wasn't removed when we dropped VBA-Next

src/BizHawk.Client.Common/movie/import/VbmImport.cs

@@ -273,7 +273,6 @@ namespace BizHawk.Client.Common.movie.import
 
 			if (isGBA)
 			{
-				Config.PreferredCores["GBA"] = CoreNames.Mgba;
 				var ss = new MGBAHawk.SyncSettings { SkipBios = true };
 				Result.Movie.SyncSettingsJson = ConfigService.SaveWithType(ss);
 			}