Remove log spam from onSubscriptionsChangedListener()

There are many bursts of 'onSubscriptionsChangedListener callback
received.' log messages that contribute greatly to log spam.  If this
message is not actively helping, then it should be removed.

Bug: 161949307
Test: this log doesn't appear
Change-Id: I9f9f2f8fa49bf50bbc9a33193d3b58813627329d