Remove test_fixup_256, which cannot detect the bug

Every child of the Node256 in test_fixup_256 is written at the same
version, so max(childMaxVersion[0..15]) and max(maxOfMax) agree and the
buggy code passes. test_fixup_256_heterogeneous_versions covers the same
Node256-under-a-range-end shape with distinct versions, so keep only that.

test_conflict_set.py

@@ -222,18 +222,6 @@ def test_merge_child_node48():
         cs.addWrites(1, write(b"\x00" * 8, b"\x00" * 10))
 
 
-def test_fixup_256():
-    # All children are written at the same version, so this test passes even
-    # if fixupMaxVersion only considers some of them. See
-    # test_fixup_256_heterogeneous_versions for that case.
-    with DebugConflictSet() as cs:
-        cs.addWrites(0, write(bytes([1])))
-        for i in range(256):
-            cs.addWrites(1, write(bytes([1, i])))
-        cs.addWrites(2, write(bytes([0]), bytes([1])))
-        cs.check(read(0, bytes([1]), bytes([2])))
-
-
 def test_fixup_256_heterogeneous_versions():
     # Regression test for issue #89: recomputing the maxVersion of a Node256
     # must consider all of its children, not just those with an index < 16.