Remove deprecated Item::GetString method

All callers have been migrated to some combination of GetIfString, GetIfToken, or GetIfByteSequence or the infallible versions of those methods.

The GetStringStrict method is not renamed in this change so that there is a pause to avoid any confusion over the associated behavior.

PiperOrigin-RevId: 967833160

quiche/common/structured_headers.h

diff --git a/quiche/common/structured_headers.h b/quiche/common/structured_headers.h
index a029e9a..0fc732c 100644
--- a/quiche/common/structured_headers.h
+++ b/quiche/common/structured_headers.h
@@ -112,9 +112,9 @@
     QUICHE_CHECK(value);
     return *value;
   }
-  // TODO(apaseltiner): Rename this to `GetString()` after all callers have been
-  // migrated off the current version of `GetString()` that works with strings,
-  // tokens, and byte sequences.
+  // TODO(apaseltiner): Rename this to `GetString()` now that all callers have
+  // been migrated off the old version of `GetString()` that worked with
+  // strings, tokens, and byte sequences.
   const std::string& GetStringStrict() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
     const auto* value = GetIfString();
     QUICHE_CHECK(value);
@@ -140,8 +140,6 @@
   const std::string* GetIfToken() const ABSL_ATTRIBUTE_LIFETIME_BOUND;
   std::string* GetIfToken() ABSL_ATTRIBUTE_LIFETIME_BOUND;
 
-  // Note: This only returns a non-nullptr if `Type() == kString`, unlike the
-  // deprecated `GetString()` and `TakeString()` methods.
   const std::string* GetIfString() const ABSL_ATTRIBUTE_LIFETIME_BOUND;
   std::string* GetIfString() ABSL_ATTRIBUTE_LIFETIME_BOUND;
 
@@ -151,25 +149,6 @@
   const bool* GetIfBoolean() const ABSL_ATTRIBUTE_LIFETIME_BOUND;
   bool* GetIfBoolean() ABSL_ATTRIBUTE_LIFETIME_BOUND;
 
-  // Deprecated: Prefer `GetStringStrict()`, `GetToken()`, or
-  // `GetByteSequence()`.
-  const std::string& GetString() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
-    struct Visitor {
-      const std::string* operator()(const std::monostate&) { return nullptr; }
-      const std::string* operator()(const int64_t&) { return nullptr; }
-      const std::string* operator()(const double&) { return nullptr; }
-      const std::string* operator()(const std::string& value) { return &value; }
-      const std::string* operator()(const Token& value) { return &value.value; }
-      const std::string* operator()(const ByteSequence& value) {
-        return &value.value;
-      }
-      const std::string* operator()(const bool&) { return nullptr; }
-    };
-    const std::string* value = std::visit(Visitor(), value_);
-    QUICHE_CHECK(value);
-    return *value;
-  }
-
   ItemType Type() const { return static_cast<ItemType>(value_.index()); }
 
  private: