Simplify cider-repl-set-ns

There's no need to actually do an eval to change the namespace of the REPL -
we only have to change it's nrepl-buffer-ns, as it's attached to all eval
requests made from the REPL buffer.

cider-repl.el

@@ -659,17 +659,6 @@ text property `cider-old-input'."
   (cider-eval-and-get-value
    "(clojure.core/map clojure.core/str (clojure.core/all-ns))"))
 
-(defun cider-repl-switch-ns-handler (buffer)
-  "Make a nREPL evaluation handler for the REPL ns switching."
-  (nrepl-make-response-handler buffer
-                               (lambda (buffer value))
-                               (lambda (buffer out)
-                                 (cider-repl-emit-output buffer out))
-                               (lambda (buffer err)
-                                 (cider-repl-emit-err-output buffer err))
-                               (lambda (buffer)
-                                 (cider-repl-emit-prompt buffer))))
-
 (defun cider-repl-set-ns (ns)
   "Switch the namespace of the REPL buffer to NS.
 
@@ -681,9 +670,8 @@ namespace to switch to."
                        (cider-current-ns))))
   (if ns
       (with-current-buffer (cider-current-repl-buffer)
-        (cider-eval
-         (format "(in-ns '%s)" ns)
-         (cider-repl-switch-ns-handler (current-buffer))))
+        (setq nrepl-buffer-ns ns)
+        (cider-repl-emit-prompt (current-buffer)))
     (error "Cannot determine the current namespace")))