[pseudo] Simplify the forest dump, NFC.

[hokein]

Summary

The code was written to handle nullable grammar, and we disallow
nullable grammar, so it is not necessary to keep it around.

clang-tools-extra/pseudo/lib/Forest.cpp

std::function<void(const ForestNode *, Token::Index, llvm::Optional<SymbolID>,
                      ? End
                      : Children[ChildIndex + 1]->startTokenIndex();
         };
         if (P->kind() == Ambiguous) {
           Children = P->alternatives();
         } else if (P->kind() == Sequence) {
           Children = P->elements();
           if (Abbreviated) {
-            if (P->startTokenIndex() == End)
-              return;
-            for (size_t I = 0; I < Children.size(); ++I)
-              if (Children[I]->startTokenIndex() == P->startTokenIndex() &&
-                  EndOfElement(I) == End) {
-                return Dump(
-                    Children[I], End,
+            if (Children.size() == 1) {
+              assert(Children[0]->startTokenIndex() == P->startTokenIndex() &&
+                     EndOfElement(0) == End);
+              return Dump(Children[0], End,
                           /*ElidedParent=*/ElidedParent.getValueOr(P->symbol()),
                           LineDec);
-              }
           }
         }
 
         if (End == KEnd)
           Result += llvm::formatv("[{0,3}, end) ", P->startTokenIndex());
         else
           Result += llvm::formatv("[{0,3}, {1,3}) ", P->startTokenIndex(), End);
         Result += LineDec.Prefix;