Update Symbols Only font preview

[why]
The Symbols Only font is listed two times on the downloads page.
It has no preview (and even no real visible name).

[how]
After changing generate-font-image-previews.sh [1] we can now generate a
preview for the Symbols Only font. Put it here manually, with up to date
fonts.json database file.

[1]:
0e434c2b9  generate-font-image-previews: Fit svg canvas to drawing [skip ci]
2597b4ef3  generate-font-image-previews: Generate Symbols Only preview

Signed-off-by: Fini Jastrow <ulf.fini.jastrow@desy.de>

_data/fonts.json

@@ -463,22 +463,12 @@
     {
       "unpatchedName": "Symbols Only",
       "RFN": false,
-      "patchedName": "Symbols-1000-em Nerd Font Complete",
+      "patchedName": "Symbols",
       "folderName": "NerdFontsSymbolsOnly",
-      "imagePreviewFont": "",
+      "imagePreviewFont": "Symbols Nerd Font Mono",
       "linkPreviewFont": false,
       "caskName": "symbols-only",
-      "description": "Just the Nerd Font Icons. I.e Symbol font only. 1000 EM size"
+      "description": "Just the Nerd Font Icons. I.e Symbol font only"
-    },
-    {
-      "unpatchedName": "Symbols Only",
-      "RFN": false,
-      "patchedName": "Symbols-2048-em Nerd Font Complete",
-      "folderName": "NerdFontsSymbolsOnly",
-      "imagePreviewFont": "",
-      "linkPreviewFont": false,
-      "caskName": "symbols-only",
-      "description": "Just the Nerd Font Icons. I.e Symbol font only. 2048 EM size"
     },
     {
       "unpatchedName": "Terminus",