OpenApiComparer is not implemented

Hello,

My team is using this repository to generate our OpenAPI document, and we started using the OpenApiComparer recently in our project. This method returns a value so we believed it was working until recently when after making a change this still returned an empty list.

After looking at the implementation it's quite clear that this method does nothing but it is documented as being a method that "Compares two OpenApiDocuments and returns a list of differences.".

I believe this method should be removed or should throw instead.

After looking a bit more, there seems to be a valid implementation of this in the vnext branch that was never merged into master.

先閱讀 src/Microsoft.OpenApi/Services/OpenApiComparer.cs，並檢查 vnext 分支中的有效實作。判斷預期的解決方案是移除該方法、讓它擲出例外，或帶入 vnext 的實作；當 OpenApiComparer 不再針對比較靜默傳回空結果時，即表示完成。