Remove restriction on maximum description length

Currently, some objects on the platform cannot be loaded as they don't respect this limit.

Reproducing:

$ beat db pull atnt/4
$ conda install bob.ip.color bob.db.atnt
$ beat db index atnt/4
Indexing database atnt/4...
Failed to load the database `atnt/4':
  * found error validating data format `system/array_1d_uint64/1' for output `template_ids' on set `probes' of protocol `idiap': ["/#description: 'Basic format containing a one-dimensional array of unsigned integer (64 bits) values' is too long (rule: /properties/#description/maxLength)"]

I wonder how it is possible to have something not respecting the core schemas in beat.web - don't we check for conformance? @samuel.gaist: would you please double-check this?