Remove unused method checkUidNetworkingBlocked

aosp/1612823 removed the usage from CS, no one is using
checkUidNetworkingBlocked() now. Thus, remove it from
NetworkPolicyManager, AIDL and NetworkPolicyManagerService.

Bug: 180084343
Test: atest FrameworksNetTests
Test: atest CtsNetTestCases
Test: atest CtsHostsideNetworkTests
Test: atest FrameworksCoreTests:NetworkPolicyManagerTest
Test: atest FrameworksServicesTests:NetworkPolicyManagerServiceTest
Change-Id: I632efa4b775a0238bb912690d48e766597e5e623