Revert "accelerate getShufflingRef (#4911)" (#4958)

This reverts commit ea97e93.

Author: etan-status

AllTests-mainnet.md

@@ -99,11 +99,10 @@ OK: 1/1 Fail: 0/1 Skip: 0/1
 OK: 2/2 Fail: 0/2 Skip: 0/2
 ## BlockRef and helpers
 ```diff
-+ commonAncestor sanity                                                                      OK
 + get_ancestor sanity                                                                        OK
 + isAncestorOf sanity                                                                        OK
 ```
-OK: 3/3 Fail: 0/3 Skip: 0/3
+OK: 2/2 Fail: 0/2 Skip: 0/2
 ## BlockSlot and helpers
 ```diff
 + atSlot sanity                                                                              OK
@@ -446,11 +445,6 @@ OK: 1/1 Fail: 0/1 Skip: 0/1
 + RestErrorMessage writer tests                                                              OK
 ```
 OK: 2/2 Fail: 0/2 Skip: 0/2
-## Shufflings
-```diff
-+ Accelerated shuffling computation                                                          OK
-```
-OK: 1/1 Fail: 0/1 Skip: 0/1
 ## Slashing Interchange tests  [Preset: mainnet]
 ```diff
 + Slashing test: duplicate_pubkey_not_slashable.json                                         OK
@@ -682,4 +676,4 @@ OK: 2/2 Fail: 0/2 Skip: 0/2
 OK: 9/9 Fail: 0/9 Skip: 0/9
 
 ---TOTAL---
-OK: 387/392 Fail: 0/392 Skip: 5/392
+OK: 385/390 Fail: 0/390 Skip: 5/390

beacon_chain/consensus_object_pools/block_dag.nim

@@ -149,30 +149,6 @@ func get_ancestor*(blck: BlockRef, slot: Slot,
 
     blck = blck.parent
 
-func commonAncestor*(a, b: BlockRef, lowSlot: Slot): Opt[BlockRef] =
-  ## Return the common ancestor with highest slot of two non-nil `BlockRef`,
-  ## limited by `lowSlot` (`err` if exceeded).
-  doAssert a != nil
-  doAssert b != nil
-  if a.slot < lowSlot or b.slot < lowSlot:
-    return err()
-
-  var
-    aa = a
-    bb = b
-  while aa != bb:
-    if aa.slot >= bb.slot:
-      aa = aa.parent
-      doAssert aa != nil, "All `BlockRef` lead to `finalizedHead`"
-      if aa.slot < lowSlot:
-        return err()
-    else:
-      bb = bb.parent
-      doAssert bb != nil, "All `BlockRef` lead to `finalizedHead`"
-      if bb.slot < lowSlot:
-        return err()
-  ok aa
-
 func atSlot*(blck: BlockRef, slot: Slot): BlockSlot =
   ## Return a BlockSlot at a given slot, with the block set to the closest block
   ## available. If slot comes from before the block, a suitable block ancestor