Remove redundant WiFi tethering listener

- WiFi tethering settings currently listeners two sets of WiFi tethering
  state from framework at the same time, There is a timing issue when
  the deprecated callback is sent earlier than the available callback,
  it will cause WiFi tethering settings to start tethering twice in a
  short period of time, causing the WiFi framework to respond with a
  startup error.

- Remove redundant obsolete WiFi tethering state listener to avoid
  WiFi tethering startup error.

Bug: 227719584
Test: manual test
make RunSettingsRoboTests ROBOTEST_FILTER=WifiTetherSettingsTest

Change-Id: I59cb5222acb763f630fb621250d7740a240bbd92