dag2dot: Remove old dax2dot

dax2dot used to be a symlink to dag2dot, but somehow it got
replaced by some old version of dag2dot. We can just remove
it and use dag2dot in every place where we would use dax2dot.
dag2dot: Improve handling of -f option

This fixes PM-721 by treating files and jobs as equivalent Nodes
so that transitive reduction works in the case where the DAG
contains a mix of File Nodes and Job Nodes. Non-redundant Job-Job
edges will still be rendered if the user specifies -f, but redundant
edges will be removed. If the user specifies both -f and -s, then
there will be many redundant edges in a typical workflow. Sometimes
the -f option will cause cycles in the graph (e.g. files with "inout"
linkage, or jobs with a file that is both an input and output). In
those cases the -s option must also be specified.