Why the RunAsync is not available on the Remove-VMHost cmdlet, I don't know.
The PowerCLI DEV Team must have had their reasons, or perhaps it is something historical.
But you can do this
$esx = Get-VMHost -Name MyEsx
$esx.ExtensionData.Destroy_Task()
The methods with _Task at the end, launch the action and then return.
They do not wait for the end of the requested action.
Blog: lucd.info Twitter: @LucD22 Co-author PowerCLI Reference