Remove feature flag "settings_dynamic_system"

The API surface of com.android.dynsystem is already protected by
privileged permission android.permission.INSTALL_DYNAMIC_SYSTEM,
which is only granted to system apps or priv apps.
Thus the feature flag check here is redundant, because eligible
callers of this API (system app) can also set the feature flag.

Bug: 176795908
Test: Presubmit
Test: Test DSU installation Intent
Change-Id: Iedac8763372ef6f11754d3f272f70753fb3d4829