osd: don't build unnecessary parts of SnapMapper for crimson

osd: don't build unnecessary parts of SnapMapper for crimson

This commits skips building two call-sites of `omap_get_iterator()`
of `ObjectStore` which are bypassing the `OSDriver` abstraction
layer.

1. `SnapMapper::convert_legacy()` likely never be needed by
crimson and we can leave it unchanged.

2. `SnapMapper::Subber` isn't needed right now but will be
needed in the future.

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>