TetherController: Remove default data limit in tethering start

Don't need anymore because the tethering control plane is moved to
the tethering service and the data limit is applied since the first
rule is added. The tethering service has the responsibility to set
the rule and the data limit now.

Bug: 150736748
Test: manual, atest netd_integration_test:NetdBinderTest#TetherOffloadForwarding
Original-Change: https://android-review.googlesource.com/1325771
Merged-In: Ie2b8d57680025be903ebfb3d33e8ef1e2e819ddd
Change-Id: Ie2b8d57680025be903ebfb3d33e8ef1e2e819ddd