How to remove "leave blank for all" message from Views exposed filter
There is a problem with view 3.13. It happened due to this commit https://www.drupal.org/node/339384#comment-10548388.

Stated as:


  Issue #339384 by colan: Added help text to keep the exposed filter
  terms list unset to keep unfiltered "Any" option default.


Apply this patch or revert your view back to version 3.11.