Simplify fork choice spec

This change should be non-substantive as any blocks in `blocks` should be descendants (inclusive) of the `store.justified_checkpoint` (refer `get_filtered_block_tree`) so that in `get_head` all blocks considered as potential heads will have `slot > justified_slot`. Considering this condition universally applies, adding the `and ...` arm to the conditional is unnecessary overhead.

specs/phase0/fork-choice.md

@@ -230,7 +230,7 @@ def get_head(store: Store) -> Root:
     while True:
         children = [
             root for root in blocks.keys()
-            if blocks[root].parent_root == head and blocks[root].slot > justified_slot
+            if blocks[root].parent_root == head
         ]
         if len(children) == 0:
             return head