Remove support for URLs that were used in past

[php-coder]

We should decommission support for URLs that were left for backward compatibility:

$ git grep -i -A1 'backward compatibility'
src/main/java/ru/mystamps/web/feature/account/AccountUrl.java:  // For backward compatibility
src/main/java/ru/mystamps/web/feature/account/AccountUrl.java-  static final String ACTIVATE_ACCOUNT_PAGE_WITH_KEY = "/account/activate/key/{key}";
--
src/main/java/ru/mystamps/web/feature/category/CategoryUrl.java:        // For backward compatibility
src/main/java/ru/mystamps/web/feature/category/CategoryUrl.java-        static final String LIST_CATEGORIES_PAGE       = "/category/list";
--
src/main/java/ru/mystamps/web/feature/collection/CollectionUrl.java:    // For backward compatibility
src/main/java/ru/mystamps/web/feature/collection/CollectionUrl.java-    static final String INFO_COLLECTION_BY_ID_PAGE = "/collection/{id}/{slug}";
--
src/main/java/ru/mystamps/web/feature/country/CountryUrl.java:  // For backward compatibility
src/main/java/ru/mystamps/web/feature/country/CountryUrl.java-  static final String LIST_COUNTRIES_PAGE     = "/country/list";
--
src/main/java/ru/mystamps/web/feature/series/SeriesUrl.java:    // For backward compatibility
src/main/java/ru/mystamps/web/feature/series/SeriesUrl.java-    static final String ADD_SERIES_WITH_CATEGORY_PAGE = "/series/add/category/{slug}";

[php-coder]

It seems like this issue obsoletes #518