test_classes: Stop using get_realm_by_email_domain.
test_classes: Stop using get_realm_by_email_domain.

get_realm_by_email_domain was intended to be registration flow code
not used in other code, but it was leaked to a few places.  This
removes one of the main remaining references to it outside the
registration code path.

eeshangarg/zulip,Galexrt/zulip,punchagan/zulip,punchagan/zulip,showell/zulip,brainwane/zulip,rishig/zulip,brainwane/zulip,mahim97/zulip,shubhamdhama/zulip,dhcrzf/zulip,eeshangarg/zulip,verma-varsha/zulip,jackrzhang/zulip,hackerkid/zulip,brockwhittaker/zulip,jackrzhang/zulip,hackerkid/zulip,zulip/zulip,hackerkid/zulip,rishig/zulip,shubhamdhama/zulip,kou/zulip,brockwhittaker/zulip,rht/zulip,timabbott/zulip,andersk/zulip,mahim97/zulip,synicalsyntax/zulip,brainwane/zulip,rishig/zulip,rht/zulip,timabbott/zulip,kou/zulip,synicalsyntax/zulip,tommyip/zulip,dhcrzf/zulip,kou/zulip,jackrzhang/zulip,mahim97/zulip,brainwane/zulip,brainwane/zulip,dhcrzf/zulip,punchagan/zulip,hackerkid/zulip,Galexrt/zulip,rht/zulip,kou/zulip,synicalsyntax/zulip,synicalsyntax/zulip,punchagan/zulip,tommyip/zulip,dhcrzf/zulip,eeshangarg/zulip,synicalsyntax/zulip,dhcrzf/zulip,jackrzhang/zulip,showell/zulip,verma-varsha/zulip,synicalsyntax/zulip,kou/zulip,Galexrt/zulip,kou/zulip,mahim97/zulip,hackerkid/zulip,amanharitsh123/zulip,andersk/zulip,brainwane/zulip,showell/zulip,timabbott/zulip,verma-varsha/zulip,mahim97/zulip,eeshangarg/zulip,jackrzhang/zulip,amanharitsh123/zulip,brockwhittaker/zulip,zulip/zulip,eeshangarg/zulip,rht/zulip,rht/zulip,dhcrzf/zulip,amanharitsh123/zulip,timabbott/zulip,punchagan/zulip,showell/zulip,brockwhittaker/zulip,brockwhittaker/zulip,showell/zulip,rht/zulip,shubhamdhama/zulip,kou/zulip,Galexrt/zulip,zulip/zulip,amanharitsh123/zulip,showell/zulip,amanharitsh123/zulip,eeshangarg/zulip,verma-varsha/zulip,punchagan/zulip,rishig/zulip,zulip/zulip,andersk/zulip,tommyip/zulip,timabbott/zulip,jackrzhang/zulip,rht/zulip,tommyip/zulip,zulip/zulip,eeshangarg/zulip,andersk/zulip,shubhamdhama/zulip,verma-varsha/zulip,brockwhittaker/zulip,andersk/zulip,brainwane/zulip,showell/zulip,shubhamdhama/zulip,rishig/zulip,jackrzhang/zulip,mahim97/zulip,dhcrzf/zulip,timabbott/zulip,tommyip/zulip,timabbott/zulip,zulip/zulip,andersk/zulip,Galexrt/zulip,tommyip/zulip,synicalsyntax/zulip,amanharitsh123/zulip,hackerkid/zulip,rishig/zulip,verma-varsha/zulip,tommyip/zulip,Galexrt/zulip,shubhamdhama/zulip,andersk/zulip,punchagan/zulip,Galexrt/zulip,shubhamdhama/zulip,hackerkid/zulip,rishig/zulip,zulip/zulip