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>
1 file changed