FIR CFA: do not remove provideDelegate subgraphs when rolling back

The receiver of the provideDelegate call is the same FirExpression as
the delegate itself, so there's only one copy of the nodes in the first
place; trying to remove subgraphs completely detaches objects inside it
from the parent graph, which is not great for checkers.

Note that currently if provideDelegate is not selected, there will be a
stray FunctionCallExit node in the control flow graph. This commit *does
not change that*. It has been there for a while. Don't @ me. I'll try to
fix that. No promises.

compiler/fir/semantics/src/org/jetbrains/kotlin/fir/resolve/dfa/cfg/CFGNode.kt

@@ -55,15 +55,6 @@ sealed class CFGNode<out E : FirElement>(val owner: ControlFlowGraph, val level:
             return true
         }
 
-        @CfgInternals
-        fun removeAllIncomingEdges(to: CFGNode<*>) {
-            for (from in to._previousNodes) {
-                from._followingNodes.remove(to)
-            }
-            to._incomingEdges = null
-            to._previousNodes.clear()
-        }
-
         @CfgInternals
         fun removeAllOutgoingEdges(from: CFGNode<*>) {
             for (to in from._followingNodes) {

compiler/fir/semantics/src/org/jetbrains/kotlin/fir/resolve/dfa/cfg/ControlFlowGraph.kt

@@ -59,16 +59,6 @@ class ControlFlowGraph(val declaration: FirDeclaration?, val name: String, val k
         _subGraphs += graph
     }
 
-    @CfgInternals
-    fun removeSubGraph(graph: ControlFlowGraph) {
-        assert(graph.owner == this)
-        _subGraphs.remove(graph)
-        graph.owner = null
-
-        CFGNode.removeAllIncomingEdges(graph.enterNode)
-        CFGNode.removeAllOutgoingEdges(graph.exitNode)
-    }
-
     private fun assertState(state: State) {
         assert(this.state == state) {
             "This action can not be performed at $this state"