Let's deprecate BLOCK_NOT_AVAILABLE_TEXT

This define contains a localized text to be shown when a block is no
longer available.
Let's deprecate it since:
1. after calling Localization::changeLocale() this define contains a text
in the wrong language.
2. there's no reason to have manage this text represented with a define
whereas all the other messages are represented directly
Merge pull request #1383 from mlocati/deprecate-useless-constants

Let's deprecate BLOCK_NOT_AVAILABLE_TEXT