Ruby: Simplify isLocalSourceNode implementation

The need for `SynthReturnNode` goes away if we don't restrict the nodes
that can't be reached from another entry definition or expression to be
`ExprNode`s

Author: RasmusWL

ruby/ql/lib/codeql/ruby/dataflow/internal/DataFlowPrivate.qll

@@ -302,13 +302,7 @@ private module Cached {
     or
     n instanceof PostUpdateNodes::ExprPostUpdateNode
     or
-    // TODO: Explain why SynthReturnNode is needed!
-    // if we don't include this, we are not able to find this call:
-    // https://github.com/github/codeql/blob/976daddd36a63bf46836d141d04172e90bb4b33c/ruby/ql/test/library-tests/frameworks/http_clients/NetHttp.rb#L24
-    n instanceof SynthReturnNode
-    or
-    // Expressions that can't be reached from another entry definition or expression.
-    n instanceof ExprNode and
+    // Nodes that can't be reached from another entry definition or expression.
     not localFlowStepTypeTracker+(any(Node n0 |
         n0 instanceof ExprNode
         or