DeleteWatchlistGroup: fix numRows() method call

[DanielWTQ]

Method was removed from the database class, call the ResultWrapper method

See T286694 for context

[DanielWTQ]

I couldn't get automated testing to confirm that this is needed, but manual testing shows that groups with edits couldn't be deleted:

{
    "error": {
        "code": "internal_api_error_Error",
        "info": "[804f2272eac3d1d41284f917] Exception caught: Call to undefined method Wikimedia\\Rdbms\\DatabaseMySQL::numRows()",
        "errorclass": "Error",
        "trace": "Error at /var/www/mediawiki/w/includes/libs/rdbms/database/DBConnRef.php(127)\nfrom /var/www/mediawiki/w/includes/libs/rdbms/database/DBConnRef.php(127)\n#0 /var/www/mediawiki/w/extensions/SemanticWatchlist/src/Api/DeleteWatchlistGroup.php(140): Wikimedia\\Rdbms\\DBConnRef->__call()\n#1 /var/www/mediawiki/w/extensions/SemanticWatchlist/src/Api/DeleteWatchlistGroup.php(46): SWL\\Api\\DeleteWatchlistGroup->deleteGroup()\n#2 /var/www/mediawiki/w/includes/api/ApiMain.php(1974): SWL\\Api\\DeleteWatchlistGroup->execute()\n#3 /var/www/mediawiki/w/includes/api/ApiMain.php(942): MediaWiki\\Api\\ApiMain->executeAction()\n#4 /var/www/mediawiki/w/includes/api/ApiMain.php(913): MediaWiki\\Api\\ApiMain->executeActionWithErrorHandling()\n#5 /var/www/mediawiki/w/includes/api/ApiEntryPoint.php(153): MediaWiki\\Api\\ApiMain->execute()\n#6 /var/www/mediawiki/w/includes/MediaWikiEntryPoint.php(200): MediaWiki\\Api\\ApiEntryPoint->execute()\n#7 /var/www/mediawiki/w/api.php(44): MediaWiki\\MediaWikiEntryPoint->run()\n#8 {main}"
    }
}

formatted trace:

Error at /var/www/mediawiki/w/includes/libs/rdbms/database/DBConnRef.php(127)
from /var/www/mediawiki/w/includes/libs/rdbms/database/DBConnRef.php(127)
#0 /var/www/mediawiki/w/extensions/SemanticWatchlist/src/Api/DeleteWatchlistGroup.php(140): Wikimedia\\Rdbms\\DBConnRef->__call()
#1 /var/www/mediawiki/w/extensions/SemanticWatchlist/src/Api/DeleteWatchlistGroup.php(46): SWL\\Api\\DeleteWatchlistGroup->deleteGroup()
#2 /var/www/mediawiki/w/includes/api/ApiMain.php(1974): SWL\\Api\\DeleteWatchlistGroup->execute()
#3 /var/www/mediawiki/w/includes/api/ApiMain.php(942): MediaWiki\\Api\\ApiMain->executeAction()
#4 /var/www/mediawiki/w/includes/api/ApiMain.php(913): MediaWiki\\Api\\ApiMain->executeActionWithErrorHandling()
#5 /var/www/mediawiki/w/includes/api/ApiEntryPoint.php(153): MediaWiki\\Api\\ApiMain->execute()
#6 /var/www/mediawiki/w/includes/MediaWikiEntryPoint.php(200): MediaWiki\\Api\\ApiEntryPoint->execute()
#7 /var/www/mediawiki/w/api.php(44): MediaWiki\\MediaWikiEntryPoint->run()
#8 {main}

Probably would have been caught by phan if this extension used that

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 10.61%. Comparing base (6e20d42) to head (6d537fe).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Api/DeleteWatchlistGroup.php	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #127   +/-   ##
=========================================
  Coverage     10.61%   10.61%           
  Complexity      429      429           
=========================================
  Files            22       22           
  Lines          1724     1724           
=========================================
  Hits            183      183           
  Misses         1541     1541           

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