Remove deprecated constructors for NetworkAgent

Finally.

Now that mLegacy is always false, removing the support
for legacy agents, a follow-up change will remove
the member and all the associated code.

Test: FrameworksNetTests NetworkStackTests
Bug: 167544279
Change-Id: I6e2c27facdd3ecc232a0aa32bf57c33cb06f118e
1 file changed