ci: do not set a ghprbPullId parameter

The GitHub Pull Requests Builder plugin sets the ghprbPullId parameter.
There is no need to make it configurable, and it even overloads the
value from the plugin by the default value.

This cauesed the jjb-validate job to run against the ci/centos branch
for each PR, not against the contents of the PR itself.

Signed-off-by: Niels de Vos <ndevos@redhat.com>

jobs/jjb-validate.yaml

@@ -10,15 +10,11 @@
           days-to-keep: 7
           artifact-days-to-keep: 7
     parameters:
-      - string:
-          name: ghprbPullId
-          description: GitHub Pull-Request ID for testing
-          default: 0
     dsl: |
       def GIT_REPO = 'https://github.com/ceph/ceph-csi'
       def GIT_BRANCH = 'ci/centos'
 
-      if (params.ghprbPullId != null && "${ghprbPullId}" != "0") {
+      if (params.ghprbPullId != null) {
           GIT_BRANCH = "pull/${ghprbPullId}/head"
       }