Delete all list items with powershell
'Add-PSSnapin microsoft.sharepoint.powershell'
try{
$w = Get-SPWeb("http://sp14-dev-02")
$l = $w.GetList("/ArcPreservationHoldLibrary")
Write-Host $l.TypeFullName
$listItems = $l.Items;
$c = $listItems.Count
while ($c--)
{
$item = $listItems.Delete($c)
Write-Host $c
}
}catch{
$ErrorMessage = $_.Exception.Message
Write-Host $ErrorMessage
}