stack/erp5: remove useless pin for jsonpickle

It is no longer used since 49cf1861 (stack/erp5: remove some unused eggs, 2022-05-18)
1 job for fix/jsonpicklepin in 0 seconds (queued for 1 second)
Status Job ID Name Coverage
  External
failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP-Python2

05:04:13