don't enable x-sendfile by default for local file store

This prevents open source installs from seeing file downloads break in
rails3. They can still set up sendfile explicitly by configuring their
web server and uncommenting the appropriate line, I've added
documentation to the production start wiki page:

https://github.com/instructure/canvas-lms/wiki/Production-Start#optimizing-file-downloads

relatedly, remove the unused send_file_or_data method.

Change-Id: I5e856b2ba50b0606330f86360d175c439ee1da50
Reviewed-on: https://gerrit.instructure.com/40357
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>
QA-Review: Brian Palmer <brianp@instructure.com>