Recent versions of Clang started warning when the loop doesn't get
vectorized, such as when compiling with -Oz (see bug).

To fix the build, remove the pragma and let the compiler decide on its
own when to vectorize.

Bug: chromium:1015665
Change-Id: I40a610c9e0d94cfd577a6cd2b01e6fdaa08bef7d
Reviewed-on: https://chromium-review.googlesource.com/c/libyuv/libyuv/+/1872580Reviewed-by: Frank Barchard <fbarchard@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>