[dynamic filtering] remove network boundary anchors

[jayshrivastava]

In #634, we added dynamic_filter_anchors.

In df55, HashJoinExec checks the plan below during execution for a dynamic filter consumer. These consumers disappear when we chop up the plan using network boundaries. The anchors are false references used to trick the dynamic filter producers into thinking consumers exist below.

This is actually fixed in apache/datafusion#24601 by making producers decide during planning to produce filter updates.