remove erroneous bitsize update

See #2261: this PR is a trivial change arising from conversation in the comments of http://trac.openmicroscopy.org.uk/ome/ticket/12126.

The upgrade script for 5.1 is in the breaking queue in https://github.com/joshmoore/openmicroscopy/commit/13750928eef094cdd05803d2d397b49d86e39e70 and the corresponding changes for `dev_5_0` are covered by #2315.