[ConstraintSystem] Finish porting member failure diagnostics.

Remove special cases for AnyObject member and dynamic member lookup
failures - these are now all handled in the new diagnostic
infrastructure.

Author: hborla

docs/TypeChecker.rst

@@ -981,10 +981,6 @@ to the new diagnostic framework, which is described in detail in this
 
 The things in the queue yet to be ported are:
 
-- Diagnostics related to member references: ``diagnoseMemberFailures``.
-  Most of the associated diagnostics have been ported and fixes are
-  located in ``ConstraintSystem::simplifyMemberConstraint``.
-
 - Problems related to calls and operator applications e.g.
 
   - Missing explicit ``Self.`` and ``self.``