doc/dev: Remove workbench mentions

doc/dev: Remove workbench mentions

This PR removes the "running-tests-in-cloud.rst"
file, which explains how to use ceph-workbench.
ceph-workbench is now deprecated, and the new
Teuthology documentation supplants the information
in the ceph-workbench-related documentation.

This PR also alters the "index.rst" file to remove
a link to "running-tests-in-cloud.rst".

Fixes: https://tracker.ceph.com/issues/49109 (partially)
Signed-off-by: Zac Dover <zac.dover@gmail.com>