Correct '-debug' command-line switch behavior

- The switch was always set, and
- -debug was not functional in the first place

Followup for https://mediawiki.org/wiki/Special:Code/pywikipedia/11397
Followup for I0d63aa63bd88050e938cb5fc47b2ca79e098fb0d
Change-Id: I45c8776b00e2ab2c1dd64055a9a62c4294e2db14