[TASK][EM] Truncate table if updating extension list

If updating the TER extension list, em deletes all rows of the table
with entries of the specific repository. Since EM handles the main
repository only, truncate can be used here, which is quicker.

Change-Id: Id0dc4fb57d68346f372c44d8cbee247072d09548
Resolves: #46569
Releases: 6.1
Reviewed-on: https://review.typo3.org/19199
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn