cubicweb #1989131 datafeed: remaining references to removed 'synchronizing' attribute [resolved]

breaks automatic import of datafeed based sources

prioritynormal
typebug
done in3.13.9
load0.100
load left0.000
closed by#b25dda2214a2 [datafeed] remove remaining uses of 'synchronizing' attribute replaced by 'in_synchronization' in 3.13.8. Closes #1989131