civicrm_import_job_* mysql tables should be cleared if the import is cancelled

When using the import tool, after the first step, a table civicrm_import_job is created in the database.
If you cancel the import, this table still exists in the database, it potentially highly increases the size of the database.

When common users uses this import tool to keep synchronization with another software, they often cancel their import for many reasons (forgot to export a field or used the wrong file ...) causing the database to blow up.... to a critical point.

Cancelling should result in a full cleaning of temp datas.