build-setup: Remove non-default DISTRO from IBM machines
build-setup: Remove non-default DISTRO from IBM machines

The meta-ibm layer has been updated to have one default DISTRO instead
of two, therefore there is no need to specify a non-default DISTRO
anymore.

Need to initialize DISTRO to empty per shellcheck:

In build-setup.sh line 332:
if [[ -z "${DISTRO}" ]]; then
          ^-------^ SC2153: Possible misspelling: DISTRO may not be
assigned, but distro is.

Change-Id: If75e9fe2c8bf1886495c583f94f8b6677d6b29a4
Signed-off-by: Adriana Kobylak <742cf4d8dead98b4cab50b088fd2a1406293c958@us.ibm.com>

openbmc/openbmc-build-scripts,openbmc/openbmc-build-scripts