test_signup: Remove unnecessary LoginOrAskForRegistrationTestCase.
test_signup: Remove unnecessary LoginOrAskForRegistrationTestCase.

This test class is basically a poor version of the end-to-end tests
that we have in `test_auth_backends.py`, and didn't really add any
value other than making it difficult to refactor.

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