How to remove 'Content Type' reference from 'Document' library programatically in SharePoint 2013

Question

I have a requirement to modify the existing 'Site column' of People/ Groups, which allows end user to chose Single user and I need to update the site column to multiple user select through the code.

I have tried updating the 'Site Column' through code but no use. After lot of research, I found it is not possible to replace existing site column (via same content type).

As a work around, we can replace the existing content type with Newly created content type containing the same set of site columns(to retain the existing list item values) & then add the required column [i.e,.,(Multiple user select) People picker column. ].

It works fine but, when I tried to delete the unused existing Site Content Type, using the below code 

SPContentType contentTypePDelete = spWeb.ContentTypes["PAGEPOLICY"];

 SPList lstPages = spWeb.Lists.TryGetList("PAGES");

if (contentTypePDelete != null)
                    { 
                        SPContentTypeId id = lstPages.ContentTypes.BestMatch(contentTypePDelete.Id);
                        //Error throwing Line
                        lstPages.ContentTypes.Delete(id);
                        //Error throwing Line
                        lstPages.Update();
}

It throws the following Error:

Content Type is still in use

What is the correct approach to remove the (existing)Content Type(which is currently not used) in Sp 2013 Document library?