[NFC] [Serialization] Avoid unnecessary check for if Identifier from AST

Inspired by the review process in
#92085.

The check `ID >= FirstIdentID` can cover the following check
`!II->isFromAST()`.

Author: ChuanqiXu9

clang/lib/Serialization/ASTWriter.cpp

@@ -3895,8 +3895,7 @@ void ASTWriter::WriteIdentifierTable(Preprocessor &PP,
 
       // Write out identifiers if either the ID is local or the identifier has
       // changed since it was loaded.
-      if (ID >= FirstIdentID || !Chain || !II->isFromAST() ||
-          II->hasChangedSinceDeserialization() ||
+      if (ID >= FirstIdentID || II->hasChangedSinceDeserialization() ||
           (Trait.needDecls() &&
            II->hasFETokenInfoChangedSinceDeserialization()))
         Generator.insert(II, ID, Trait);