Remove empty MNC filtering when retrieving available subscriptions

During the early phase of work on DSDS, we found that a hacky solution
to seeing duplicate subscriptions that are part of a logical group (eg
Google Fi) was to filter out the entries with empty MNC strings. It
turns out this breaks things for some legitimate eSIM subsctiptions, and
isn't needed anymore because of the fixes that landed for b/127833821.

Fixes: 130706446
Test: make RunSettingsRoboTests
Change-Id: Ic4472c1de3c3e2449139b3e2ffb20d11a0058e8a