Remove obsolete Smart Lock developer settings

There is no code that changes the trust_agents_extend_unlock or
lock_screen_when_trust_lost settings from their default values.  These
settings were introduced as a "temporary convenience for dogfooding"
when the "extend unlock" feature was initially added in Android 10
(https://android.googlesource.com/platform/frameworks/base/+/15f7dd0c29881228b40d6db7f7f6e5d151cdb832).

Before Android 10 was released, extend unlock became the default
(https://android.googlesource.com/platform/frameworks/base/+/d3268c2dca1f09b6cef01579588b7aeb7c867cb9).

In Android 11, the ability to toggle these settings was removed from the
Settings app
(https://android.googlesource.com/platform/packages/apps/Settings/+/638aa7c066c4046ebc092fcfe4b29eb35040fbfc).

Therefore, there's no need to support anything other than the default
values of these settings.  Simplify the code accordingly.  Note, no
tests need to be removed because the non-default values were untested.

Bug: 120871688
Test: atest TrustTests
Change-Id: I6697b578712dfb57f9e709c541454b43e79d3a14