Revert "Improve theme handling to support themes using `spacemacs-theme` custom api"

This reverts commit 5f308b0306.

This message is shown on startup:
spacemacs/startup-hook: Symbol’s value as variable is void: spacemacs-theme-custom-colors
and it broke several things.

core/core-spacemacs.el

@@ -221,10 +221,6 @@ Note: the hooked function is not executed when in dumped mode."
                              "Calling dotfile user config...")
      (dotspacemacs|call-func dotspacemacs/emacs-custom-settings
                              "Calling dotfile Emacs custom settings...")
-     (defconst spacemacs-original-spacemacs-custom-colors spacemacs-theme-custom-colors
-       "Value of spacemacs-theme-custom-colors as it was after running user-config.
-
-Needs to be saved as some themes customise this variable and we can otherwise not restore the original setting.")
      ;; don't write custom settings into the dotfile before loading them,
      ;; otherwise https://github.com/syl20bnr/spacemacs/issues/10504 happens
      (spacemacs/initialize-custom-file-sync)

core/core-themes-support.el

@@ -370,7 +370,6 @@ THEME."
   "Cycle through themes defined in `dotspacemacs-themes'.
 When BACKWARD is non-nil, or with universal-argument, cycle backwards."
   (interactive "P")
-  (setq spacemacs-theme-custom-colors spacemacs-original-spacemacs-custom-colors)
   (let* ((themes (if backward (reverse dotspacemacs-themes) dotspacemacs-themes))
          (next-theme (car (or (cdr (memq spacemacs--cur-theme themes))
                               ;; if current theme isn't in cycleable themes, start