Remove a superfluous warning in LUNodeRemove

Since we run the post-hooks explicitly in the Exec() function (via
_RunPostHook) after we removed the target node from the config, we
will get a:

  WARNING Node 'node2', which is about to be removed, was not found in
  the list of all nodes

in the logs every time we remove a node. The patch just removes the
warning, as actually invalid configurations (for the pre hook) will be
checked correctly elsewhere.

Additionally, the docstrings for BuildHooksEnv and BuildHooksNodes are
corrected/switched.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>