Revert "Add explicit synchronization point."

This reverts commit 4ee78ca7.

- that patch had a number of problems:  (i) it is inappropriate for the writing routine to have the side effect of altering the graph structure, this breaks the primary use case of editing existing DAGs, wherein one loads a DAG and then write back to disk the *same* DAG;  (ii) the particular change that was introduced requires the resulting DAG to have an extra submit file that is not provided, resulting in DAGs that cannot be run;  (iii) the code would insert no-op jobs even between a single parent and its children or an only child and its parents if there are more children or parents than some threshold, increasing the size of the DAG (the opposite of what this is supposed to be doing), and because the transformation is embedded in the writing code the expansion would be repeated every time the DAG is processed;  (iv) the problem this is trying to fix is not in this code at all but in the inspiral DAG construction script, which is creating bloated parent-child relationships and is where this fix, if desired, should have been made.
- assuming one actually wants a tool to do this to DAGs, say, instead of fixing the programs that generate the bloated DAGs, the correct solution is to write a separate utility to implement the transformation, and not embed it in the writing code.  this patch provides that tool.  it is left as an exercise to others to modify their clean-up programs to make use of the tool, but, again, just fix the DAG generator.