Remove unused permission from NetworkStackTests

Since TetheringCoverageTests would include NetworkStackTestsLib and
tethering has unit test case that tests permission.
That test would assume the test package do not define some permission
and use adopt/drop shell permission to simulate whether caller hold
testing permission.

Remove it to avoid to be inherited by its user(TetheringCoverageTests).

Bug: 154869719
Test: TetheringCoverageTests, NetworkStackTests, NetworkStackNextTests
Change-Id: Iac917ddaad32ae4549b7f8d376de8492a958fbeb