Revert "Add read_channel_content permission (#24018)" (#24114)

This reverts commit a19ce047ba.

server/channels/app/app_test.go

@@ -112,7 +112,6 @@ func TestDoAdvancedPermissionsMigration(t *testing.T) {
 	expected1 := map[string][]string{
 		"channel_user": {
 			model.PermissionReadChannel.Id,
-			model.PermissionReadChannelContent.Id,
 			model.PermissionAddReaction.Id,
 			model.PermissionRemoveReaction.Id,
 			model.PermissionManagePublicChannelMembers.Id,