runtime: remove flaky "goroutine 2 bt" from gdb test

runtime: remove flaky "goroutine 2 bt" from gdb test

This part of the test has been flaky despite repeated attempts to fix it,
and it is unclear what exactly it is testing. Remove it.

Fixes #24616.

Change-Id: If7234f99dd3d3e92f15ccb94ee13e75c6da12537
Reviewed-on: https://go-review.googlesource.com/c/go/+/233942
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>