Remove extraneous fourthTry

If `canInstantiate` fails, we fallback to `compareLower` which itself
can fallback to `fourthTry`, so there's no need to fallback to
`fourhTry` inside `canInstantiate` itself.

Author: smarter

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -1028,8 +1028,7 @@ class TypeComparer(using val comparerCtx: Context) extends ConstraintHandling wi
 
       /** `param2` can be instantiated to a type application prefix of the LHS
        *  or to a type application prefix of one of the LHS base class instances
-       *  and the resulting type application is a supertype of `tp1`,
-       *  or fallback to fourthTry.
+       *  and the resulting type application is a supertype of `tp1`.
        */
       def canInstantiate(tycon2: TypeParamRef): Boolean = {
         def appOK(tp1base: Type) = tp1base match {
@@ -1051,8 +1050,7 @@ class TypeComparer(using val comparerCtx: Context) extends ConstraintHandling wi
                   false
               }
               liftToBase(tp1w.baseClasses)
-            } ||
-            fourthTry
+            }
         }
       }