exec.cpp:exec: Don't remove block_io elements from j->io

When dulicate_prepend prepend copies of block_io elements to j->io, the
removing simply does nothing. Now that the same pointers are prepended to
j->io, don't remove them.
Revert "Revert shared_ptr<io_data_t> changes until kinks are ironed out"

This reverts commit 77f1b1f0fe27778750bb9b9aa53f6bc42d5e5843.