[MusicXML import] multiple staff-details element result in hidden brackets

File beethoven_op2no1psu.xml (see #19612: Attached MusicXML causes crash) contains many staff-details elements. Due to a bug in the MusicXML importer, for each staff-details element, an invisible brackets are added to each system. Even though the brackets are not visible, the layouter apparently still reserves some horizontal space for them, which adds up to a large empty space at the left side of the page.