[CodeCompletion] Remove completeCaseStmtDotPrefix() completion

Now that, normal dot-member completion works at case position. We don't
need to handle it specially.

rdar://problem/45219937

include/swift/Parse/CodeCompletionCallbacks.h

@@ -177,9 +177,6 @@ public:
   /// Complete at the beginning of a case stmt pattern.
   virtual void completeCaseStmtBeginning() {};
 
-  /// Complete a case stmt pattern that starts with a dot.
-  virtual void completeCaseStmtDotPrefix() {};
-
   /// Complete at the beginning of member of a nominal decl member -- no tokens
   /// provided by user.
   virtual void completeNominalMemberBeginning(