Remove Tethering @TestApi

Modules shouldn't have TestApis, as documented in go/android-api-types.
Additionally, nothing depends on these TestApis existing.

Bug: 170395679
Test: m checkapi
Change-Id: I6e2c8298e90b4b54f0264be974d036fa08cd5632
Merged-In: I6e2c8298e90b4b54f0264be974d036fa08cd5632