mfd: qcom-pm8xxx: Drop unused PM8018 compatible

The PM8018 compatible is always used with PM8921 fallback, so PM8018
compatible can be safely removed from device ID table

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20220928-mdm9615-dt-schema-fixes-v4-8-dac2dfaac703@linaro.org