Refactor: remove unnecessary check code in AliasThis.semantic

It was a workaround code for the AST internal inconsistency which has caused by template instantiations.

Now, it's not necessary anymore, because the problem is properly handled at the top of StructDeclaration.semantic.

    ```d
    if (type.ty == Tstruct && (cast(TypeStruct)type).sym != this)
    {
        TemplateInstance ti = (cast(TypeStruct)type).sym.isInstantiated();
        if (ti && isError(ti))
            (cast(TypeStruct)type).sym = this;
    }
    ```

src/aliasthis.d

@@ -70,20 +70,12 @@ public:
                 .error(loc, "undefined identifier %s", ident.toChars());
             return;
         }
-        else if (ad.aliasthis && s != ad.aliasthis)
+        if (ad.aliasthis && s != ad.aliasthis)
         {
             .error(loc, "there can be only one alias this");
             return;
         }
 
-        if (ad.type.ty == Tstruct && (cast(TypeStruct)ad.type).sym != ad)
-        {
-            AggregateDeclaration ad2 = (cast(TypeStruct)ad.type).sym;
-            assert(ad2.type == Type.terror);
-            ad.aliasthis = ad2.aliasthis;
-            return;
-        }
-
         /* disable the alias this conversion so the implicit conversion check
          * doesn't use it.
          */