signup: Remove old frontend validation of subdomain.
signup: Remove old frontend validation of subdomain.

We make request to check availibity of subdomain which also checks the
subdomain for illegal characters.  Duplicating the backend logic in
frontend in this case is thus no longer necessary.

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