migration: 17->18: Drop the thumbnail linking table as well

Since we're in a weak sqlite context, the table won't be emptied
through the foreign key constraint, causing the table to be in an
inconsistent state.
5 jobs for fix_17_18_migration in 20 minutes and 50 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #291014
amd64 debian
asan-ubsan

00:10:20

passed #291011
amd64 debian
linux

00:09:59

passed #291012
win32
win32

00:12:06

passed #291015
win64
win64

00:08:44

failed #291013
win64
win64

00:06:00