The current code is needlessly checking the number of bytes needed to
serialize the unspentness bitmap in the utxo against a maximum value
that could never be returned because the function takes a uint32 output
index which is treated as a bit offset, and converts it bytes, which
will necessarily be less than a max uint32.

This check also causes a compile error on arm where native integers are
32 bits.

This simply removes the unneeded check.

Merges in 5a1e77b from btcd.