bitbake-layers: fix duplicated help info

There are two "help" lines when run bitbake help under python 2.6.6
(perhaps all python 2.6):

$ bitbake-layers help
[snip]
help
    display general help or help on a specified command
[snip]
help
    display general help or help on a specified command

This is because the cmd.py in python 2.6.6 also has a do_help() function
as our bitbake-layers, and the cmd.Cmd.get_names() doesn't consider the
overload, this has been fixed in python 2.7, use set() to remove the
duplicated would fix the problem.

[YOCTO #3428]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>