Fix edge removal in the CallGraph class + improve runtime a bit

[marcus-h]

First commit: Fix edge removal in the CallGraph class

If CallGraph.removeAllEdgesOutOf or CallGraph.removeEdges is called,
the corresponding edge(s) is not entirely removed from the CallGraph
data structures.

• removeAllEdgesOutOf(Unit u):
  The to be removed edges are not removed from the call graph data structures.
  For each to be removed edge e, the code first calls e.remove(), which sets
  the Edge.invalid attribute to true. Afterwards, removeEdge(e, false) is
  calledonly executed. The edge removal code in removeEdge(...) is only
  executed, if the edge e is contained in the edges set. Since Edge.invalid
  was set to true, e.hashCode() returns the constant value 0. Since the return
  value of e.hashCode() was different before Edge.invalid was set to true, the
  edge cannot be found in the edges set. Hence, no edge removal code is
  executed.
  In order to fix this, do not call e.remove() before calling removeEdge(...).
  Moreoever, the call to e.remove() is not needed because it is already called
  in removeEdge(...).

• removeEdges(Collection edges):
  First, this.edges.removeAll(edges) is executed and afterwards, for each
  edge e in the passed edges collection, removeEdge(e, false) is executed.
  Since the edge is not contained anymore in the this.edges set, no edge
  removal code in removeEdge(...) is executed.
  In order to fix this, do not call this.edges.removeAll(edges).

The expected behavior is documented in the CallGraphTest test case.

Second commit: Improve the runtime of CallGraph.removeAllEdgesOutOf a bit

Instead of iterating over all callgraph edges to find all edges that
originate from a specific unit, simply call edgesOutOf(...) (which
internally uses a LinkedHashMap for the initial lookup).
This seems to cause a noticeable speedup in UnreachableCodeEliminator.

[StevenArzt]

Thank you for the good analysis and the fix.