Revert "Avoid calling |Convert.convert| around |Ops.index| in some cases"

It turned out the problem is more complicated and one probably cannot do
the correct conversion in |Ops.index| based on the type of the default
alone. We need to investigate this more. Meanwhile, let's revert the
commit that introduced this feature.

This reverts commit a361c61e54b80f11e9d60fc6bb149d41d4d1c323.