Remove deprecated plexus/DirectedGraph

[yurishkuro]

Hard to tell what was the intention/distinction between plexus/Digraph and plexus/DirectedGraph, but there seemed to be an intention to remove it:

• the latter is not used anywhere in jaeger-ui or internally in plexus (other than being exported)
• the history log shows changes by @tiffon to DirectedGraph stopped but changes to Digraph continued
• there are comments in several places "Update import after killing DirectedGraph"

Open issue: plexus/demo does use DirectedGraph

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.47%. Comparing base (25d13e3) to head (29b9651).
Report is 311 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1940   +/-   ##
=======================================
  Coverage   96.47%   96.47%           
=======================================
  Files         256      256           
  Lines        7600     7600           
  Branches     1976     1976           
=======================================
  Hits         7332     7332           
  Misses        268      268           

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[yurishkuro]

@hari45678 would you like to revisit and finish this one?

[hari45678]

@hari45678 would you like to revisit and finish this one?

Sure. would look into it.