I just tried to do the same thing on Jenkins 2.5 but there the remote trigger option is gone...
Use the LTS Jenkins Version with the actual Pipeline plugins and create a new Pipeline-Multibranch item using the Pipeline script from SCM definition with the URL https://github.com/reinhapa/junit-compat.git
Set the Trigger builds remotely (e.g., from scripts) option under Build Triggers
After saving and re-open the build trigger setting for remote triggering is no longer active.
Used Versions:
Jenkins 1.642.2
Pipeline-Multibranch: 1.13
Is this a standalone project (thus, bug), or a branch project (thus, possibly duplicate of
JENKINS-34005)? Steps to reproduce from scratch? Exact plugin versions?