MEDIUM: Remove option lb-agent-chk

MEDIUM: Remove option lb-agent-chk

Remove option lb-agent-chk and thus the facility to configure
a stand-alone agent health check. This feature was added by
"MEDIUM: checks: Add agent health check". It will be replaced
by subsequent patches with a features to allow an agent check
to be run as either a secondary check, along with any of the existing
checks, or as part of an http check with the status returned
in an HTTP header.

This patch does not entirely revert "MEDIUM: checks: Add agent health
check". The infrastructure it provides to parse the results of an
agent health check remains and will be re-used by the planned features
that are mentioned above.

Signed-off-by: Simon Horman <horms@verge.net.au>