Cross fingers and remove code that zeroes the nursery when doing LDV
profiling.  This was required for two reasons:

  - the compiler might leave slop between objects when it is
    allocating.  However, it doesn't do this any more: we always
    move the heap pointer back at the end of a basic block if it
    turns out we didn't need all the heap,

  - After a heap check failure, we don't know what the previous
    value of Hp was, so we don't know where the end of the real
    data in the current nursery block is.

    This isn't the case any more, because as a side effect of the
    fixes to support big-block allocation we always shift Hp back
    by the required amount after a heap check failure, so now
    bd->free always points to the byte after the last allocated
    byte in a nursery block.