Allow turning off nerd fonts without restarting

Not a terribly important feature for users, probably, but it is useful for
developers when testing a new feature with or without nerd fonts; and it does go
against or policy of all configs being hot-reloadable.

Note that switching from nerd fonts version 2 to 3 is *not* supported without
restarting, but I find this less important.

pkg/gui/gui.go

@@ -489,6 +489,8 @@ func (gui *Gui) onUserConfigLoaded() error {
 		icons.SetNerdFontsVersion(userConfig.Gui.NerdFontsVersion)
 	} else if userConfig.Gui.ShowIcons {
 		icons.SetNerdFontsVersion("2")
+	} else {
+		icons.SetNerdFontsVersion("")
 	}
 
 	if len(userConfig.Gui.BranchColorPatterns) > 0 {