tests: Removed test-progs/pthread

tests: Removed test-progs/pthread

The Pthread source is now found in gem5-resources:
https://gem5.googlesource.com/public/gem5-resources/+/refs/heads/master/src/pthreads

The pre-compiled binaries are pulled from dist.gem5.org.

There is no reason to keep these here. They are therefore being
removed.

Change-Id: Id07dc72c62d19920fb304eea505deaa9ebc5599f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33141
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>