.gitattributes

branchcache: drop the unused `_verifyclosed`

This code appears dead since its introduction about 5 years ago in this three
consecutive commits:

- 6578654916ae → introduce the method with two calls
- 7c9d4cf23adf → remove first call
- be5eeaf5c24a → remove second call


    o  changeset:   be5eeaf5c24a
    |  user:        Pulkit Goyal <[e-mail]>
    |  date:        Fri Apr 05 15:57:09 2019 +0300
    |  summary:     branchcache: don't verify closed nodes in _branchtip()
    |
    o  changeset:   7c9d4cf23adf
    |  user:        Pulkit Goyal <[e-mail]>
    |  date:        Fri Apr 05 15:56:33 2019 +0300
    |  summary:     branchcache: don't verify closed nodes in iteropen()
    |
    o  changeset:   6578654916ae
    |  user:        Pulkit Goyal <[e-mail]>
    ~  date:        Mon Apr 01 13:56:47 2019 +0300
       summary:     branchcache: lazily validate nodes from the branchmap

# So GitLab doesn't think we're using tons of Perl
*.t -linguist-detectable