Feature ID: 14285.

This Workaround works only from version 10.5. Also take a backup before implementing the steps.
Bulk mark default technician option:
1->
Go to Admin -> technician list view -> Select All Accounts and
All Sites -> Bulk select Technicians which has to be marked as
default technician -> Action -> Associate to Site -> A popup to
select site will appear.
2-> Open debug tool ->Add following script in console and press ENTER
jQuery("#Site_Id").append('<option value="null">defaultSite</option>');
jQuery("#Site_Id").val("null");
3->In
the associate site popup defaultSite will be selected -> Click on
Associate -> All the selected technician in list view will be marked
as default technician.
1-> Go to Admin -> technician list view -> Select All Accounts and All Sites -> Bulk select Technicians which has to be removed as default technician -> Action -> Dissociate from Site -> A popup to select site will appear.
2-> Open debug tool (F12 in chrome or right click-> inspect) ->Add following script in console and press ENTER
jQuery("#Site_Id").append('<option value="null">defaultSite</option>');
jQuery("#Site_Id").val("null");
3->In the dissociate site popup defaultSite will be selected -> Click on Dissociate -> All the selected technician in list view will be removed as default technician.