Remove echo statement

Sphinx already prints 'The HTML pages are in docs/_build/html.' at the
end of a build. This is redundent and not cross platform compatible
(fails on Windows).

Change-Id: I6e4098aaaed492138d8056495c6db69d3799de1a
Signed-off-by: Thanh Ha <thanh.ha@linuxfoundation.org>