Admin checks: remove alternating rows logic

Following CSS changes introduced with ModernUI, the alternative row
logic was left in place, resulting in CSS class names generated with a
'1' or '2' suffix (e.g. alert-danger1, alert-info2). Since these classes
do not actually exist, the result table cells were all displayed with
white background instead of using a color matching the test result.

Since we don't need alternating row colors anymore, the whole logic has
been removed from Check API.

Fixes 0021599