[SE] Remove Utils directory

Summary:
There is no purpose in splitting out the Error class from the rest of
the StreamExecutor code. This organization was just a vestige of an old
failed design.

Plus, this change fixes a bug in the build where the utilites library
was not being statically linked in with libstreamexecutor.

Reviewers: jlebar, jprice

Subscribers: beanz, parallel_libs-commits

Differential Revision: https://reviews.llvm.org/D24434

llvm-svn: 281118