Seems to be a new thing in 5.5. Never noticed it before, but was a fun exercise for me. This is written to change ALL VM's. There is no checking to see whether the box is checked or what the reservation is set at. With that said, below will probably get you on the right path.
$VMs = Get-Cluster | Get-VM
Foreach ($VM in $VMs)
$configspec = New-Object VMware.Vim.VirtualMachineConfigSpec
$configspec.MemoryAllocation = New-Object VMware.Vim.ResourceAllocationInfo
# This will uncheck the box.
$configspec.MemoryReservationLockedToMax = $false
# This will put the reservation back to 0
$configspec.MemoryAllocation.Reservation = 0