[WIFI] Rename restart callback unregistration method

Harmonize with the registration method name.

Bug: 175084231
Bug: 180007424
Test: atest com.android.server.wifi
Test: atest android.net.wifi
Test: atest android.net.wifi.cts.WifiManagerTest#testRestartWifiSubsystem
Change-Id: Ie713d94a79362306f3be6ba03acba256ef9c255e