Revert "[libc++] Run the builders Docker containers 'as 'buildbot instead of 'root'"

This reverts commit a32b94c.

The buildbot startup scripts need to run as root. The buildbot
worker should have already been running as a different account.
More investigation needed.

Author: EricWF

libcxx/utils/docker/debian9/buildbot/Dockerfile

@@ -36,7 +36,5 @@ RUN ln -s /opt/llvm-tot/bin/clang /usr/local/bin/clang
 RUN ln -s /opt/llvm-tot/bin/clang++ /usr/local/bin/clang++
 
 
-USER buildbot
-
 ADD run_buildbot.sh /
 CMD /run_buildbot.sh /run/secrets/buildbot-auth