[BUGFIX] Removing single category item not possible

The "items" selector in a system categories misses a "size"
property. It is thus considered by the forms JS to be a single
select field and all items are removed at once rather
than the selected one.

Resolves: #53665
Releases: 6.2, 6.1
Change-Id: Ice539a42b60a14fd604a9b43ecb4dd48b85d3ae3
Reviewed-on: https://review.typo3.org/27016
Reviewed-by: Francois Suter
Tested-by: Francois Suter
Reviewed-on: https://review.typo3.org/27026