Remove explicit garbage collection from LockSettingsService

Remove the explicit garbage collection from LockSettingsService, since
it has been causing performance problems and is no longer useful.

The explicit GC was added several years ago by commit c6f4ebf7
(http://ag/12000923) which had the following explanation:

    This is to sanitize memory containing sensitive user lockscreen
    credentials. Most of LockSettingsService already sanitizes
    credentials when needed, but there is one copy of the credential
    unmarshalled from the binder transaction and passed into LSS as
    argument which is not easily sanitiziable manually except by forcing
    a garbage collection.

However, LockSettingsService has since been updated to be more thorough
in its zeroization.  This includes zeroizing LockscreenCredential
objects that were unmarshalled from Binder transactions, and zeroizing
internally-retrieved unified profile passwords.  Also, the explicit GC
never guaranteed zeroization in the first place.

Test: atest FrameworksServicesTests:com.android.server.locksettings
Bug: 320392352
Bug: 416768837
Flag: EXEMPT bugfix
Change-Id: I92dcb4d98801f3b97f45909bd7b8e18000af5a00