WifiManager: Mark disableEphemeralNetwork as @hide

This is only used by settings, so no need to make it @SystemApi.
Removing the @SystemApi tag which was unnecessarily added in ag/5529624.

Bug: 122278104
Test: make system-api-stubs-docs-update-current-api
Change-Id: I74a67abdc5e1ed2f2f049e560a3e6f8a1d0e8d76