Less navigation; same destination...

Less navigation; same destination... (Remove NavigationAccessor and its cache)

We were previously looking at the CLR type of the navigation to determine whether it was a collection or not. Now we're using the IsUnique metadata from the foreign key. This means that the CLR type info caching for navigations is not required anymore.
We decided some time back to not add these tests anymore because the overall value proposition was negative. However, we haven't been removing ones that are there. Also, if we ever move to a more sophisticated method of generating the checks using Roslyn, then we will likely need to start adding these tests again because, as with the Code Contracts approach, a Roslyn approach will, at least initially, likely have bugs around the non-obvious cases.