Clean up unused NetworkPolicyEditor

From DataUsageSummaryPreferenceController.

NetworkPolicyEditor.read() is not called, so
NetworkPolicyEditor.getPolicy() will always returns null.

Data limit info is already set in getDataUsageInfo().

Bug: 295260929
Test: robo test
Change-Id: I56a40cc906aa95207785886e704b1a59ea344e9c