user [nfc]: Inline store.userDisplayEmail, which has become trivial

For #236 matching users by email address, we'd like to add some
fields to AutocompleteDataCache, and that'll be simpler if we don't
have to involve the whole UserStore.

Author: chrisbobbe

lib/model/user.dart

@@ -91,12 +91,6 @@ mixin UserStore on PerAccountStoreBase, RealmStore {
     return getUser(senderId)?.fullName ?? message.senderFullName;
   }
 
-  /// The user's real email address, if known, for displaying in the UI.
-  ///
-  /// Returns null if self-user isn't able to see the user's real email address,
-  /// or if the user isn't actually a user we know about.
-  String? userDisplayEmail(int userId) => getUser(userId)?.deliveryEmail;
-
   /// Whether [user] has passed the realm's waiting period to be a full member.
   ///
   /// See:

lib/widgets/autocomplete.dart

@@ -285,7 +285,7 @@ class MentionAutocompleteItem extends StatelessWidget {
         label = store.userDisplayName(userId);
         emoji = UserStatusEmoji(userId: userId, size: 18,
           padding: const EdgeInsetsDirectional.only(start: 5.0));
-        sublabel = store.userDisplayEmail(userId);
+        sublabel = store.getUser(userId)?.deliveryEmail;
       case WildcardMentionAutocompleteResult(:var wildcardOption):
         avatar = SizedBox.square(dimension: 36,
           child: const Icon(ZulipIcons.three_person, size: 24));

lib/widgets/profile.dart

@@ -56,7 +56,7 @@ class ProfilePage extends StatelessWidget {
       .merge(weightVariableTextStyle(context, wght: 700));
 
     final userStatus = store.getUserStatus(userId);
-    final displayEmail = store.userDisplayEmail(userId);
+    final displayEmail = user.deliveryEmail;
     final items = [
       Center(
         child: Avatar(