test_classes: Remove DEFAULT_REALM variable.
test_classes: Remove DEFAULT_REALM variable.

This essentially unused legacy variable was causing Zulip to query the
database at import time, which is generally not something we aim to
do.

Combined with the issue fixed in the previous commit, this variable
resulted in test-backend providing an unhelpful crash when provision
hadn't updated the unit testing database.

synicalsyntax/zulip,andersk/zulip,timabbott/zulip,shubhamdhama/zulip,synicalsyntax/zulip,zulip/zulip,punchagan/zulip,punchagan/zulip,rht/zulip,shubhamdhama/zulip,eeshangarg/zulip,hackerkid/zulip,kou/zulip,brainwane/zulip,hackerkid/zulip,timabbott/zulip,hackerkid/zulip,hackerkid/zulip,synicalsyntax/zulip,kou/zulip,punchagan/zulip,rht/zulip,zulip/zulip,brainwane/zulip,showell/zulip,brainwane/zulip,punchagan/zulip,synicalsyntax/zulip,punchagan/zulip,timabbott/zulip,eeshangarg/zulip,synicalsyntax/zulip,synicalsyntax/zulip,eeshangarg/zulip,eeshangarg/zulip,hackerkid/zulip,rht/zulip,punchagan/zulip,showell/zulip,showell/zulip,kou/zulip,hackerkid/zulip,timabbott/zulip,shubhamdhama/zulip,shubhamdhama/zulip,zulip/zulip,punchagan/zulip,showell/zulip,showell/zulip,eeshangarg/zulip,rht/zulip,zulip/zulip,zulip/zulip,timabbott/zulip,andersk/zulip,showell/zulip,rht/zulip,kou/zulip,rht/zulip,shubhamdhama/zulip,zulip/zulip,showell/zulip,eeshangarg/zulip,zulip/zulip,timabbott/zulip,kou/zulip,kou/zulip,andersk/zulip,timabbott/zulip,andersk/zulip,rht/zulip,hackerkid/zulip,kou/zulip,andersk/zulip,brainwane/zulip,andersk/zulip,shubhamdhama/zulip,shubhamdhama/zulip,brainwane/zulip,andersk/zulip,eeshangarg/zulip,brainwane/zulip,brainwane/zulip,synicalsyntax/zulip