Drop pull-artifact-files config option for remote execution

With `pull-artifact-files` disabled, artifact file blobs are completely
ignored in cache checks. While this allows remote execution builds
without downloading build outputs, it can result in build failures when
blobs expire on the server side. It also does not download missing blobs
on demand. I.e. `bst artifact checkout` and other commands don't work
with `pull-artifact-files` disabled.

This commit removes the `pull-artifact-files` config option due to the
issues above. It will be replaced by remote cache support, which only
downloads blobs as needed but still considers all file blobs in cache
checks for reliable builds. It will also support download of missing
blobs on demand.
5 files changed