slugbuilder: Revert "build as root if BUILD_AS_ROOT is set"

This reverts commit a2e5abb6094d3f0e833f1ef1a2a0356a001c295b.

This feature was unused, and didn't fully realize the intended
goals. Originally merged in #552.

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>
Merge pull request #632 from flynn/revert-slugbuilder-root

slugbuilder: Revert "build as root if BUILD_AS_ROOT is set"