spacemacs/set-font: Remove deprecated function

This function has been deprecated since 2015, surely it is fine to
remove by now.

core/core-fonts-support.el

@@ -118,14 +118,6 @@ The return value is nil if no font was found, truthy otherwise."
  powerline-scale 1)))
  (truncate (* scale (frame-char-height)))))
 
-(defun spacemacs/set-font (&rest args)
- "Deprecated function, display a warning message."
- (spacemacs-buffer/warning (concat "spacemacs/set-font is deprecated. "
- "Use the variable `dotspacemacs-default-font' "
- "instead (see Font section in "
- "~/.emacs.d/doc/DOCUMENTATION.org for more "
- "info).")))
-
 (defmacro spacemacs|diminish (mode &optional unicode ascii)
  "Diminish MODE name in mode line to UNICODE or ASCII depending on the value
 `dotspacemacs-mode-line-unicode-symbols'.