emoji [nfc]: Make realmEmoji internal to EmojiStoreImpl

It's a bit of a trap for the unwary (... like me last month, in
ecd2cb5, causing #1113), because it includes deactivated realm emoji
as well as active ones.

lib/model/emoji.dart

@@ -102,10 +102,6 @@ final class EmojiCandidate {
 
 /// The portion of [PerAccountStore] describing what emoji exist.
 mixin EmojiStore {
-  /// The realm's custom emoji (for [ReactionType.realmEmoji],
-  /// indexed by [Reaction.emojiCode].
-  Map<String, RealmEmojiItem> get realmEmoji;
-
   EmojiDisplay emojiDisplayFor({
     required ReactionType emojiType,
     required String emojiCode,
@@ -135,7 +131,8 @@ class EmojiStoreImpl with EmojiStore {
   /// The same as [PerAccountStore.realmUrl].
   final Uri realmUrl;
 
-  @override
+  /// The realm's custom emoji (for [ReactionType.realmEmoji],
+  /// indexed by [Reaction.emojiCode].
   Map<String, RealmEmojiItem> realmEmoji;
 
   /// The realm-relative URL of the unique "Zulip extra emoji", :zulip:.

lib/model/store.dart

@@ -386,9 +386,6 @@ class PerAccountStore extends ChangeNotifier with EmojiStore, ChannelStore, Mess
   ////////////////////////////////
   // The realm's repertoire of available emoji.
 
-  @override
-  Map<String, RealmEmojiItem> get realmEmoji => _emoji.realmEmoji;
-
   @override
   EmojiDisplay emojiDisplayFor({
     required ReactionType emojiType,

test/model/store_checks.dart

@@ -33,7 +33,6 @@ extension PerAccountStoreChecks on Subject<PerAccountStore> {
   Subject<String> get zulipVersion => has((x) => x.zulipVersion, 'zulipVersion');
   Subject<int> get maxFileUploadSizeMib => has((x) => x.maxFileUploadSizeMib, 'maxFileUploadSizeMib');
   Subject<Map<String, RealmDefaultExternalAccount>> get realmDefaultExternalAccounts => has((x) => x.realmDefaultExternalAccounts, 'realmDefaultExternalAccounts');
-  Subject<Map<String, RealmEmojiItem>> get realmEmoji => has((x) => x.realmEmoji, 'realmEmoji');
   Subject<List<CustomProfileField>> get customProfileFields => has((x) => x.customProfileFields, 'customProfileFields');
   Subject<int> get accountId => has((x) => x.accountId, 'accountId');
   Subject<Account> get account => has((x) => x.account, 'account');