Revert "transient--show: Fix only window height"

This reverts commit 03997dc.

This avoids an issue for the `transient-posframe' package, mentioned
at 03997dcba8d5301b6eb2675.

It is unknown why fixing only the height caused the height to be
reduced, but since we fixed both the height and width for a long time,
without *that* causing any issues, just go back to doing that.

lisp/transient.el

@@ -3590,7 +3590,7 @@ have a history of their own.")
                              (button-get (1- (point)) 'command))
                         (transient--heading-at-point))))
       (erase-buffer)
-      (setq window-size-fixed 'height)
+      (setq window-size-fixed t)
       (when (bound-and-true-p tab-line-format)
         (setq tab-line-format nil))
       (setq header-line-format nil)