This was a copy-and-paste of subprocess.check_output() from when we supported
Python <2.7, so simply delete it and use subprocess.check_output() instead.
(From OE-Core rev: b1f2d9ed8d4dc89c9e669f43f546463ccc2a76b9)
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>