Don't bother showing .tree in search result paths

[eslavich]

The search function returns paths like this:

["root.tree['meta']['wcs']",
 "root.tree['meta']['wcsinfo']",
 "root.tree['meta']['cal_step']['assign_wcs']",
 "root.tree['meta']['ref_file']['specwcs']",
 "root.tree['meta']['wcsinfo']['wcsaxes']"]

When I wrote that code, I didn't notice that this is also possible:

["root['meta']['wcs']",
 "root['meta']['wcsinfo']",
 "root['meta']['cal_step']['assign_wcs']",
 "root['meta']['ref_file']['specwcs']",
 "root['meta']['wcsinfo']['wcsaxes']"]

(The AsdfFile class implements a __getitem__ that fetches from the tree)

We ought to update the search paths to the more succinct option.