Fixes ISSUE-54687: Remove training widget (obsolete content)

The content of this widget we show by default in backoffice workspace is no longer kept outdated or relevant (to show there). Remove

• widget class + url entry
• widget instances for the active usages
• old auto-generated _ACCESS rows related to it

try:

• https://builds.openbravo.com/view/try/job/try-init/3305
• all good except api, which is handled here: openbravo/ci/backoffice-api!63 (merged)

try-retail:

• https://builds.openbravo.com/job/try-ret-init/10803/
• good (2 false positives inside pos1 UI)
• Run needed to confirm install.source with extra sampledata worked (see openbravo/ci/modules/org.openbravo.test.mobile.sampledata!120)