test: Drop sandbox_set_enable_memio() from mux-cmd test

This test does not appear to use sandbox's memory-mapped I/O so there is
no need to enable it.

Even if there were a need, it should be disabled at the end of the test,
so as not to affect other tests.

Drop these lines from the test.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/dm/mux-cmd.c b/test/dm/mux-cmd.c
index 6eb3b28..bd02cda 100644
--- a/test/dm/mux-cmd.c
+++ b/test/dm/mux-cmd.c
@@ -26,8 +26,6 @@
 	int i;
 	unsigned long val;
 
-	sandbox_set_enable_memio(true);
-
 	ut_assertok(uclass_get_device_by_name(UCLASS_MUX, "a-mux-controller",
 					      &dev));
 	chip = dev_get_uclass_priv(dev);
@@ -119,8 +117,6 @@
 	char cmd[BUF_SIZE];
 	unsigned int i, state;
 
-	sandbox_set_enable_memio(true);
-
 	ut_assertok(uclass_get_device_by_name(UCLASS_MUX, "a-mux-controller",
 					      &dev));
 	chip = dev_get_uclass_priv(dev);
@@ -153,8 +149,6 @@
 	char cmd[BUF_SIZE];
 	unsigned int i, state;
 
-	sandbox_set_enable_memio(true);
-
 	ut_assertok(uclass_get_device_by_name(UCLASS_MUX, "a-mux-controller",
 					      &dev));
 	chip = dev_get_uclass_priv(dev);