mypy: Change realm.subdomain to no longer be Optional.
mypy: Change realm.subdomain to no longer be Optional.

It hasn't been optional since we removed REALMS_HAVE_SUBDOMAINS.

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