mod_dav_svn: Do not set Last-Modified response header for GET responses.
mod_dav_svn: Do not set Last-Modified response header for GET responses.

This saves a bit of unnecessary work on the server-side, since:
- This header is not used by Subversion clients
- We allow caching GET responses for up-to 1 week without re-validation
- Browsers and proxies support ETag and use it for re-validation instead of
  the Last-Modified header

See the discussion in thread "Last-Modified HTTP header in GET responses" [1]

[1] https://www.mail-archive.com/dev@subversion.apache.org/msg34354.html

* subversion/mod_dav_svn/repos.c
  (get_last_modified): Remove.
  (set_headers): Do set Last-Modifed response header.


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@1724790 13f79535-47bb-0310-9956-ffa450edef68

YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion