remove old call to deactivate-mark

I believe this is a leftover from when we used to use regions to
"highlight" symexes. Nowadays we use an overlay for this, so there's
no case I can think of (currently - in the future we may have
selection of regions via #53) where the region would be active while
in Symex mode.

symex-misc.el

@@ -489,7 +489,6 @@ ORIG-FN applied to ARGS is the invocation being advised."
 
 (defun symex-exit-mode ()
   "Take necessary action upon symex mode exit."
-  (deactivate-mark)
   (when symex--original-blink-cursor-state
     (blink-cursor-mode 1))
   (when symex-refocus-p