Remove NETWORK_STACK_UID check

NETWORK_STACK_UID check was introduced by aosp/1134635, and it
assumed that wifi factories/agents will be running on the network
stack process, but it didn't eventually. It runs in the system
server instead. So remove this check.

Bug: 170598012
Test: atest FrameworksNetTests
Change-Id: Ie11268738e364e252f18876f7a4a0dc064ca8b9c