Terminal setting to X11 fails in macOS, and is not required

on Linux.
Setting terminal here is redundant, so remove this setting.

src/frontend/plotting/gnuplot.c

@@ -385,18 +385,11 @@ void ft_gnuplot(double *xlims, double *ylims,
     }
 
     /* Set up the file header. */
-#if !defined(__MINGW32__) && !defined(_MSC_VER) && !defined(__CYGWIN__)
-    fprintf(file, "set terminal X11 noenhanced\n");
-#elif defined(__CYGWIN__)
 #ifndef EXT_ASC
-        fprintf(file, "set encoding utf8\n");
-#endif
-#else
-#ifndef EXT_ASC
-        fprintf(file, "set encoding utf8\n");
+    fprintf(file, "set encoding utf8\n");
 #endif
     fprintf(file, "set termoption noenhanced\n");
-#endif
+
     if (contours) {
         fprintf(file, "set view map\n");
         fprintf(file, "set contour\n");