Add "Remove Agent" function

[Geertvdc]

PR Summary

Added a function to remove vsts agents from a pool. by calling Remove-Agent or Remove-VSTeamAgent

PR Checklist

• Write Help
• Write Unit Test
• Update VSTeam.psd1
• Update README.md

i didn't update the readme. thought it was something to be added when you do a new release right?

[DarqueWarrior]

Please add

[CmdletBinding(SupportsShouldProcess = $true, ConfirmImpact = "Medium")]

And add a force switch parameter.

if ($force -or $pscmdlet.ShouldProcess($item, "Remove Agent")) {

You can look at Set-VSTeamApproval for an example. Otherwise looks great. Thanks for the support.

[Geertvdc]

I've updated these two things