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.

Test: manual, atest netd_integration_test:NetdBinderTest#TetherOffloadForwarding
Change-Id: Ie2b8d57680025be903ebfb3d33e8ef1e2e819ddd