Revert "Add grammar fields to GenerationConfig for constrained decoding" (pytorch#19620)

This reverts commit 7355d7b.

### Summary
Temporarily reverting to restore test health for QNN jobs.

Author: GregoryComer

extension/llm/runner/irunner.h

@@ -33,14 +33,6 @@ struct GenerationConfig {
   // Whether to echo the input prompt in the output
   bool echo = true;
 
-  // Grammar definition for constrained decoding (e.g. a JSON schema, regex,
-  // Lark CFG, or GBNF grammar). Empty string means no constraint.
-  std::string grammar;
-
-  // Grammar format: "json_schema", "regex", "lark", or "gbnf".
-  // Only used when grammar is non-empty.
-  std::string grammar_type;
-
   // Whether to ignore EOS token and continue generating until max_new_tokens
   bool ignore_eos = false;