Drop FileChecker

[RKrahl]

As already discussed during the collaboration meetings and announced via the mailing list: it seems that nobody is actually using the FileChecker. In that case, it is better to remove that feature.

In principle, it is certainly a good idea to regularly verify that the files in the storage are accessible and have correct checksums. But this may be better done using a separate script that would iterate over the datafiles in icat.server and checks them in the storage. There is no need to have that integrated in ids.server and implementing that separately makes it easier to custumize the behavior according to the facility's needs.

According to current plans, we will deprecate that feature in ids.server 2.0.0 and remove it in 3.0.0.