[Sb refactor] Remove throwing condition when requesting a repo

Initially it seemed unlikely that we would ever request a mobile
connections repository for a subscription id that we had not-yet been
told about. I.e., a view model requesting state for subId N should only
happen after we are told that N is an existing subscription.

This check is fine in the majority of cases, but it's prone to race
conditions since to check if a subscription is valid we need to check
the _current_ list of subscriptions.

Removing this thrown exception since it doesn't meet the criteria of
catching actionable exceptions, nor does it represent an actual bad
state anymore. If we ever end up with a view model tracking a
subscription that does not exist, then it will show up in logs and we
can trace where it came from then.

Test: all tests in com.android.systemui.statusbar.pipeline
Fixes: 273863830
Change-Id: I2453b5ebc08f738e6cf25689860558a7a5c47c44