MDEV-22500 Assertion `thd->killed != 0' failed in ha_maria::enable_indexes

For MDEV-17223 the issue was an assert that didn't take into account that
we could get duplicate key errors when enablling unique indexes.
Fixed by not retrying repair in case of duplicate key error for this
case, which avoids the assert.

For MDEV-22500 I removed the assert, as it's not critical (just a way to
find potential wrong code) and we will anyway get things logged in the
error log if this happens. This case cannot triggered an assert in 10.3
but I verified that it would trigger in 10.5 and that this patch fixes
it.