Remove synchronization from WorkflowRun.doStop

[jglick]

Revealed by a CloudBees CI test failing after jenkinsci/workflow-api-plugin#368:

java.lang.AssertionError: Synchronizing on WorkflowRun before metadataGuard may cause deadlocks
	at PluginClassLoader for workflow-job//org.jenkinsci.plugins.workflow.job.WorkflowRun.getMetadataGuard(WorkflowRun.java:219)
	at PluginClassLoader for workflow-job//org.jenkinsci.plugins.workflow.job.WorkflowRun.doKill(WorkflowRun.java:495)
	at PluginClassLoader for workflow-job//org.jenkinsci.plugins.workflow.job.WorkflowRun.httpKill(WorkflowRun.java:513)
	at PluginClassLoader for workflow-job//org.jenkinsci.plugins.workflow.job.WorkflowRun.doStop(WorkflowRun.java:892)

Here getOneOffExecutor() is null immediately after startup, before the build has had a chance to resume.

I am not sure if this could have happened previously in realistic timing conditions, perhaps under more exotic conditions involving unloadable builds, but anyway from code inspection doStop was indeed acquiring a lock in the wrong order when this clause was hit: this code path could not have worked (for years at least, did not look up how long exactly).