Cleanup: src.ctf.fs: index no longer used in loop

This removal should have been done in an earlier commit modifying this
loop:
  Author: Francis Deslauriers <francis.deslauriers@efficios.com>
  Date:   Tue Apr 30 19:01:17 2019 -0400

      src.ctf.fs: compute stream range using entire file info group

      Given that index entries are sorted within their file info structure and
      that file info structures are sorted by time within the file info group,
      we can simply compute the range of the stream by using the absolute
      first index entry and the absolute last index entry of the entire file
      info group.

Add `BT_ASSERT()` to enforce invariants in developer mode.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I4e0b53cbb808835886b97b6d6001ecd7bcc2abb4
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1288
Reviewed-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>