universal-access: Remove redundant label-for relations

The appropriate relation is already set with the mnemonic_widget property.

In particular, this also fixes an issue where a screenreader would
emit "Crosshairs" every time a new tab was entered in the Zoom dialog
because of some incorrect labelled-by relations.
11 jobs for universal-a11y in 3 minutes and 34 seconds (queued for 3 seconds)