Fix purpose and magit: max-lisp-eval-depth

problem:
trying to open a magit buffer after
reloading the configuration: `SPC f e R`

shows the message:
>Lisp nesting exceeds ‘max-lisp-eval-depth’

cause:
purpose-x-magit-multi-on can't be called twice,
without turning it off first: purpose-x-magit-off

Spacemacs, purpose-x-magit-multi-on, Lisp nesting exceeds ‘max-lisp-eval-depth’
https://github.com/bmag/emacs-purpose/issues/178

solution:
in this case we'll only call: purpose-x-magit-multi-on
once per Emacs session.

notes:
this also removes the call to: with-eval-after-load 'magit
because it's handled upstream:
c85dd3c9f7/window-purpose-x.el (L243)

layers/+spacemacs/spacemacs-purpose/packages.el

@@ -157,5 +157,5 @@
       (purpose-x-golden-ratio-setup)
 
       ;; Show magit-log-select and diff in two windows
-      (with-eval-after-load 'magit
+      (unless purpose-x-old-magit-display-buffer-function
         (purpose-x-magit-multi-on)))))