cylc dependency graph: fix handling pre-initial over-bracketing.

This fixes a bug where unnecessarily bracketed clauses cause a crash
when they contain pre-initial cycle references. This is because the
unnecessary brackets in the graph persist through to the data structure
that is fed to the cleanup algorithm. These are now removed.
cylc dependency graph: remove debug statements
Merge pull request #822 from benfitzpatrick/dep-syntax.fix-pre-initial-overly-bracketed

cylc dependency graph: fix handling pre-initial over-bracketing.