Removing apparently senseless exception.

=== Starting buildShellScriptOnSlave(org.jenkinsci.plugins.workflow.job.WorkflowTest)
.. hudson.slaves.CommandLauncher launch
INFO: slave agent launched for slave0
Exception in thread "Executor #0 for slave0 : executing org.jenkinsci.plugins.workflow.support.steps.ExecutorStep$PlaceholderTask$PlaceholderExecutable@c455a14" java.lang.IllegalStateException: Trying to remove unknown computer
	at hudson.model.AbstractCIBase.removeComputer(AbstractCIBase.java:149)
	at hudson.model.Computer.removeExecutor(Computer.java:837)
	at hudson.model.Executor.run(Executor.java:263)