Simplify Commit List sorting logic

[bobanm]

Some minor improvements to Commit List sorting logic:

1. ascendingHeight is not an appropriate argument for the argument passed to CommitList.getAll(), where the argument defines sorting order.
2. Instead of having 2 different sort commands in CommitList.getAll() based on argument value, this could be reduced to 1 command with a smarter use of COMMIT_SORT constant values.
3. It's not needed to use sort order argument in CommitPool._getAllCommits(), as ascending is the default sort order of CommitList.getAll().
4. CommitList.deleteByHeight() has an unusual conditional, which checks for an element which was deleted on the very previous line, and then tries to delete it again [?!].