Don't pass the Data Release when constructing MTL filenames

[geordie666]

As part of #684, we changed the data model such that the directory path to MTL files no longer includes the Data Release. This broke the MTL-specific read routines, such as desitarget.io.read_targets_in_tiles(..., mtl=True) which were still expecting to need to pass around Data Release names.

This is a very minor PR to fix that bug.

[geordie666]

@araichoor: Thanks for finding this bug when reading MTL ledgers and reporting it to me. I think this PR will fix your issues. If this does work, then please feel free to merge this PR (when tests pass) if it's a blocking factor for your work.

Alternatively, I can merge this PR myself when I get out of meetings in an hour or two.

Also: Let me know if you need me to make a new tag of desitarget in the short-term so that you can make "official" use of this update.

[coveralls]

Coverage increased (+0.005%) to 59.633% when pulling 3e7459c on ADM-mtl-bup-read into 594191d on master.

[araichoor]

I confirm it runs fine now; I let you do the merging.
about a new tag: right, that may be a cleaner way to proceed (but won t change the files); if that s not too much hassle for you, yes.