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 <anoo@us.ibm.com>
diff --git a/build-setup.sh b/build-setup.sh
index fc9aaaf..4ea6355 100755
--- a/build-setup.sh
+++ b/build-setup.sh
@@ -143,17 +143,7 @@
 chown "${UID}:${GROUPS[0]}" "${xtrct_path}"
 
 # Perform overrides for specific machines as required.
-case ${target} in
-  witherspoon-tacoma)
-    DISTRO="openbmc-openpower"
-    ;;
-  rainier)
-    DISTRO="openbmc-openpower"
-    ;;
-  *)
-    echo "Unspecified machine ${target}; default to local.sample.conf values."
-    ;;
-esac
+DISTRO=${DISTRO:-}
 
 # Set build target and BitBake command
 MACHINE="${target}"