Remove legacy network factories

Nothing on the system is using registerNetworkFactory,
unregisterNetworkFactory at the moment.

registerNetworkFactory, unregisterNetworkFactory are protected by
signature permissions, so could not be used by anything outside of the
system.

Remove the two methods and the underlying support for this legacy,
deprecated mechanism.

Bug: 179229316
Test: atest FrameworksNetTests
Change-Id: I7cdc9eed67f846c8774474af038133040aeccab3