papr: use new PAPR_PULL_TARGET_BRANCH

PAPR now provides pull request tests with the branch that the test is
targeting. Let's use that instead of querying the GitHub API and
possibly running through the unauthenticated limit.

.papr.sh

@@ -1,22 +1,6 @@
 #!/bin/bash
 set -xeuo pipefail
 
-pip install requests
-
-query_github() {
-    repo=$1; shift
-    resource=$1; shift
-    python -c "
-import sys
-import requests
-j = requests.get('https://api.github.com/repos/$repo/$resource').json()
-for q in sys.argv[1:]:
-    if q.isdigit():
-        q = int(q)
-    j = j[q]
-print(j)" "$@"
-}
-
 # Essentially use a similar procedure other openshift-ansible PR tests use to
 # determine which image tag should be used. This allows us to avoid hardcoding a
 # specific version which quickly becomes stale.
@@ -24,9 +8,7 @@ print(j)" "$@"
 if [ -n "${PAPR_BRANCH:-}" ]; then
     target_branch=$PAPR_BRANCH
 else
-    # check which branch we're targeting if we're a PR
-    target_branch=$(query_github $PAPR_REPO pulls/$PAPR_PULL_ID base ref)
-    [ -n "$target_branch" ]
+    target_branch=$PAPR_PULL_TARGET_BRANCH
 fi
 
 # this is a bit wasteful, though there's no easy way to say "only clone up to