libsnapshot: GetReminingTime not required after mapping dm-user

After MapDmUserCow, there is no further calls in this function. Snapshot
is mapped; Hence, checking remaining time is not required as
it may return false if the time was exceeded. This would mean function returning false even though snapshots were successfully mapped.

Bug: 291288998
Test: OTA

Change-Id: I4fd8fa6ef90a48885392297f217361507268ba51
Signed-off-by: Akilesh Kailash <akailash@google.com>