Do not set microversion for server diagnostics negative test

After microversion 2.48, response data from each hypervisor
differs from that before 2.48, so microversion is set in
test_server_diagnostics.py, but for server diagnostics negative
test("Non-admin user cannot view server diagnostic"), there
is no difference between before and after 2.48, so no need to
set microversion for server diagnostics negative test.

Change-Id: I0c256aa58258de934fd6e49cc5e1ef6b8ab1abd6
1 file changed