Remove unused enum TextRun::TextCodePath

New Clang versions warn that kAuto shadows LengthType::kAuto. It's easy to fix
because the enum isn't used at all.

Bug: 895475
Change-Id: I04c63a9ff87afdb08b060a987bc7a160fecc62e4
Reviewed-on: https://chromium-review.googlesource.com/c/1283691
Commit-Queue: Hans Wennborg <hans@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600065}