LeetCode: Remove Duplicates from Sorted Array
There is not much to review here. This algorithm looks to be as good as it gets. I don't know how much value there is in nitpicking, but here I go:

Review


The summary tag should be used to describe a type or a type member. Use remarks to add supplemental information (xmldoc summary) -> put the challenge URL in a remarks tag, not in the summary.
The challenge provides an input parameter nums but C# naming conventions don't invite you to use abbreviations. You could/should change its name to numbers.
Your unit test contains a variable with a typo excpected. 
Your method is declared public, meaning you should check arguments against null to avoid the nasty NullReferenceException.
You've added alot of inline comments to show how the algorithm works. Considering the simplicity of the algorithm and the presence of unit tests, I feel these comments don't add much value to the code, if any at all //else continue to next item etc.