More swarm_bot cleanup.
More swarm_bot cleanup.

Remove stale iteration limiting options.
Remove unused support for 'check_requirements.py'.

R=csharp@chromium.org
BUG=swarming:109

Review URL: https://codereview.appspot.com/97710043

pombreda/swarming,madecoste/swarming,luci/luci-py,madecoste/swarming,pombreda/swarming,luci/luci-py,madecoste/swarming,pombreda/swarming,luci/luci-py,luci/luci-py,pombreda/swarming,madecoste/swarming