Fix Bug, Unable to find type [PSPlot] in ISE

When loading cmdlet info from ISE "Commands" panel you get the error
"Unable to find type [PSPlot]"
https://github.com/dfinke/ImportExcel/issues/131

ImportExcel.psm1

@@ -30,8 +30,6 @@ if ($PSVersionTable.PSVersion.Major -ge 5) {
     . $PSScriptRoot\Plot.ps1
 
     Function New-Plot {
-        [OutputType([PSPlot])]
-        Param()
 
         [PSPlot]::new()
     }