AzDevOpsProject: Remove unnecessary property declaration

[johlju]

Suggest we should remove the Mandatory from the key property as Key is implicitly mandatory.

AzureDevOpsDsc/source/Classes/010.AzDevOpsProject.ps1

Line 27 in f1d8097

[DscProperty(Key, Mandatory)]

[johlju]

This is just a style guideline thing. Tracking it in issue #29 too.