AST: Rip out "defaulted definitions" from NormalConformance, NFC

It appears we were only using this to see if an associated type was
derived or defaulted. This code didn't mesh well with the other stuff
I was doing for default implementations, so I'd rather rip it out and
just rely on calling 'isImplicit' to check for derived associated
types instead.

Note that there's a small change of behavior -- if an associated type
is derived for one conformance, and then used as a witness in another,
we were previously only marking it as defaulted in the first one,
but now it is marked as defaulted in both. I do not believe this has
any meaningful consequences.

include/swift/Serialization/ModuleFormat.h

@@ -1135,10 +1135,8 @@ namespace decls_block {
     BCVBR<5>, // value mapping count
     BCVBR<5>, // type mapping count
     BCVBR<5>, // inherited conformances count
-    BCVBR<5>, // defaulted definitions count
     BCArray<DeclIDField>
-    // The array contains archetype-value pairs,
-    // then type declarations, then defaulted definitions.
+    // The array contains archetype-value pairs, then type declarations.
     // Inherited conformances follow, then the substitution records for the
     // associated types.
   >;