salad: minor bps tree code cleanup

 - Add bps_tree_delete_value to the comment and declarations.
   (All other public methods are there.)
 - Fix typo in a comment: approxiamte -> approximate.
 - Fix comment to bps_tree_random.
 - Remove repeated word 'count' from comments.

NO_DOC=no change
NO_TEST=no change
NO_CHANGELOG=no change