selftests: mount: remove no longer needed config option

Since commit eedf265a ("devpts: Make each mount of devpts an
independent filesystem.") CONFIG_DEVPTS_MULTIPLE_INSTANCES isn't needed
in the defconfig anymore.
Signed-off-by: NAnders Roxell <anders.roxell@linaro.org>
Signed-off-by: NShuah Khan (Samsung OSG) <shuah@kernel.org>

tools/testing/selftests/mount/config

 CONFIG_USER_NS=y
-CONFIG_DEVPTS_MULTIPLE_INSTANCES=y