Remove `src/plugins/ctf/common/bfcr/btr.gdb`

Remove `src/plugins/ctf/common/bfcr/btr.gdb`

We don't use nor maintain that. I'm not even sure what it does. We used
it mostly while developing Babeltrace 2.0, but things are stable now.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I3ced5c5e57ee55013820cac77cdb456f3f984b0a
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11375
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>