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: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>